The Physiology of Beatboxing
Many people think of their voice as one sound; one voice. A human beat box looks at the voice's components and learns to control them independently.
First in line is the diaphragm. Here is where you take air in and push it back out. Listen to the sound you make as you breathe in and out. You are using two different elements: the diaphragm and the larynx. The larynx can do a whole slew of things, and can be controlled independently of the diaphragm. With your larynx you make pitch, and sometimes just breath noise.
Next up is the back of your tongue. This is where you make snare drums, or any other sound with a "K" or "G" in them; side sticking, for instance. The tip of the tongue and teeth make certain snares and cymbals. By manipulating the volume of resonating space in your mouth you can change certain elements of your sound: adding a filter sweep, for instance. The tongue and lips make all sorts of sounds, most commonly the bass drum: an unvoiced "D" or "B". The lips themselves can make noise: I use mine to create that low buzz bass sound. It's just like playing a tuba.
You can teach yourself to control these parts individually, and add them together in unusual ways; humming a note with your larynx while buzzing another with your lips, while tapping out cymbals with your tongue on the back of your teeth and driving the whole rhythm from your diaphragm, for instance.
The second aspect of human beatbox is in the perception. The human ear can only hear so many things at once, and the brain blends everything without our even knowing it. It's like a flip-book that's made up of stationary figure illustrations, but when you rapidly thumb through it, it becomes a cartoon because the eye is not fast enough to keep up. The ear does the same thing. If I imply certain lines, the audience will hear them even if I don't actually do them. I can also cheat on rhythm because the ear wants to hear it. I can put a plosive on a note that is cheated ever so slightly ahead of the beat, say a word on the back of the beat, and the audience will hear them as simultaneous occurrences - a word and a bass drum at the same time, for instance.
So go ahead. Try imitating sounds or making noises that no one else has before. It's fun, it's cheap, and it's a great party trick!