When a new student comes to his/her first lesson, one of the most frequent answers to the question, "Why are you seeking voice training?", is that the singer wants to learn how to sing high(er) and/or loud(er). That is understandable; audiences react strongly to high & loud singing, and it has been very prevalent in popular music for decades. In fact, it has become synonymous with "great singing ability". Some singers can do this naturally, but most singers who want to have this skill come to voice lessons. So here's what you need to know about acquiring (and keeping) that particular skill.
Now, for most singers, their request comes from an inability to sing upwards without an audible crack or flip in the middle of their range. They usually try to circumvent this problem by either a) singing louder and louder as they go up to try and "push" past the breaking point, but sacrificing upper range and comfort, or b) switching to a much lighter voice early on in their range, but sacrificing volume and presence, aka "power". So let's take a closer look at "range" and "power", and the sustainability of this type of singing.
The "flip" or "crack" most untrained singers experience in the middle of their range is nothing unusual or worrisome, in fact that break will be erased completely with consistent voice training. Your voice teacher will work with you to learn how to optimize the way you inhale and exhale in order to sing well, and how to keep the vocal cords engaged properly, while teaching you how to relax all the upper body muscles that like to mess with your vocal cords' freedom by tensing around the larynx (your voice box, the part of your neck that moves up and down when you swallow).
It just so happens that your voice box has a double duty in our bodies. Its primary function is to act as a valve to make sure that no solid or liquid particles enter your airways. If something goes wrong, well, you know how it feels when you swallow and it "goes down the wrong way"! In that case, the voice box needs to get whatever went down to your lungs back out, and fast. How does it do that? By triggering a coughing fit. Everything, including the vocal cords, shut down hard so you can build up air pressure inside your lungs, and then you force the air out with each cough until the intruder is cleared from your lungs. If your larynx didn't know how to handle its primary job, you wouldn't survive long.
But it also has a secondary job, which is to produce sound so we can communicate verbally. This does not require anywhere close to the level of air pressure that we use for coughing or sneezing, for example. In fact, the voice cannot function properly with insufficient or excessive air pressure. There is a small range of usable air pressure for talking/singing, and when you cross the upper limit, your voice box has to work so hard to try and hold back the air to produce sound that it literally squeezes shut, and your vocal cords can no longer vibrate to make sound. It calls on the help of all kinds of upper body muscles to do this, and the singer feels like he/she is "hitting a wall". And by now you've probably figured out that "pushing harder" to get past it, is futile.
It's easy to see how this works. Just make a lip bubble, or lip trill, aka "blow a raspberry". If you don't blow enough air, your lips won't vibrate. If you blow too much air, you either blow your lips apart or have to press them together so hard that they're not free to vibrate anymore.
So the first priority while learning how to keep your voice from cracking or flipping, is to learn how to maintain everything around the voice box as relaxed as possible, particularly your tongue, jaw, and neck. This needs to be done at a lower volume at first, so the larynx doesn't switch over to survival mode. The priority here is freedom (effortless singing), not power. That will come later. The thing is, this first step takes time. Have patience during this process, and it will pay off in a big way! (there are ways to cheat around the break if you have an audition tomorrow, but those "quick fixes" shouldn't be taken as a permanent way to sing.)
Congratulations, you've been patient, and your crack is gone for good! Or is it? Does it come back when you attempt to get loud? Imagine that instead of taking voice lessons, you were going to the gym to strengthen your arm muscles. Heck, if you want to really "belt it out", you're going for body-builder arms! You're probably going to put waaaay too much weight on your bar, and use all your back muscles in addition to your not-yet-strong biceps to lift the weight. Your trainer will advise you to back off the weight and start with something almost comically light until you learn the proper form (no back muscle engagement, only biceps). Once you're mastered it, you will add a little more weight, and begin this process again until your back muscles stop engaging. It's how you build up your strength in the right places while avoiding serious injury.
With singers, this process is quite similar. Think of the biceps as your vocal cords, your back muscles as the upper body muscles, especially around the neck and mouth, and the amount of weight as your volume. By now you've learned how to keep everything relaxed while singing at low volume, so your next goal will be a medium volume, and then, finally, your loud volume. You have to take it one step at a time to find out where your personal air pressure threshold is, and if you try to go from zero to sixty too fast, you are most likely to cross it, and trigger the shutdown. It's easy to test if you're ready to go louder on any given note: if you can only sing it loudly (that is, push to get it), but not softly, you're doing it wrong.
Notice how I said your loud volume and your personal threshold. Every body is built differently, and some singers' anatomical configurations lend themselves better for high & loud singing than others'. All vocal ranges and dynamic abilities are not created equally. If this were the case, singers who could do this would be the norm, not the exception, and the ability to belt wouldn't be anything special. A capable voice teacher knows this and works with you to reach realistic goals within your given anatomical configuration. They will help you find "your sound" by suggesting the genre and vocal style that best suits your voice, so you can show off what you can do rather than what you can't. There are highly successful singers across all genres and styles, so "high & loud" isn't the only qualifier for great singing. For example, both Céline Dion and Janet Jackson have had very successful careers, but they are very different singers.
Think of the combination of high and loud as your fastest sprint, your heaviest weight, or your highest jump. The younger you are, the fitter you are, and the more natural endurance you have, the longer you will be able to do this. But even professional athletes don't do this all the time. The runner leaves her fastest speed for the end of the race, the bodybuilder lifts his heaviest weight only in competition, and the jumper won't attempt the highest jump at every training session, but rather focus on form. They cannot operate at 100% peak efficiency and skill all the time.
Like any race, a belter's every song builds up to a highlight. So if you hit your first chorus at your highest and loudest, you have nowhere to go as the song progresses. Nor does every song on your album or your concert set list have to be at your highest and loudest. This can be quite tiring for your audience's ears, not to mention your vocal cords. Your goal as a performer should be to move your audience emotionally, and this can be done in many different ways. Less is more, more often than not.
Assuming that you are one of the singers who can sing this way, you have to be careful though. Remember this skill is your
voice at maximum output, so you shouldn't overuse it. The vocal cords are made of very soft tissue, and in order to produce high and loud notes, they must be stretched out quite a bit, and collide hundreds, if not thousands of times per second for every single one of those notes. No matter how great your technique, pace yourself, or you will suffer the consequences over time. Just like a runner needs to rest their muscles after each race, singers need 24 hours to be back at 100% after a concert. If you've overused your high and loud, and you happen to be under 30, then you'll most likely recover within that timeframe, but as you age, the body takes longer to recover. Which increases the chance of you singing again on a not-fully-recovered voice, and then you start a vicious cycle of vocal cord swelling that can lead to serious vocal injury. You just can't retain the "youth brilliance" in your voice if you keep stretching your vocal range and dynamic abilities beyond limits. I've witnessed too many singers (who became famous at an early age for their "high and loud" skills) progressively lose their vocal abilities over time, and those who have poor technique end up with serious vocal trouble too early in their careers. Granted, the music industry is also partially responsible with impossible demands in terms of schedule and rest for the artists, but I can assure you that a technically savvy singer who doesn't stretch to the max on each song will have less trouble in the long run.