That's a great question (and it's something that I used to do all the time when I was younger!).

In our inner ears (inside our eardrums in our heads), we have a series of canals that give us information about the position of our head in space. It's how you know which way your head is moving (like to the right or left, up or down), even if our eyes are closed.

Basically, these canals are arranged in half-circles and are filled with a liquid. The insides of the canals (the part with the liquid) have lots of little hair-like projections, each of which is in turn connected to a nerve cell. When we move our head, the liquid in the canals moves as well, and the movement of the liquid moves the hair-like projections, sort of the like how the hair on your arms moves when you put them under running water.

As the hair-like projections move in the liquid, they trigger the nerves. The nerves then send all of this information to the brain, and the brain processes all of it and gives you the sensation of knowing which way your head is moving. Pretty cool, huh?

Anyway, this is what happens when we are moving around normally. When you spin your head around a lot, the liquid in these canals get moving really fast too, moving all of the hair-like projections very fast, triggering all of the nerves all at once and over and over again. Pretty soon everything is moving so fast that the nerve cells and the brain can't keep up with all of the information from the motion. Basically, you have overloaded the ability of the nerve cells and the brain to process all of this information. The brain can't deal with all the information it's getting, and you feel dizzy.

Since your brain isn't sure which way is up, it can't tell your body which muscles it needs to use to stand upright, you fall down. As you lie on the ground, the nerve cells and the brain recover (helped by the fact that the liquid has finally stopped moving so fast) and you begin to correctly feel which way is up again.

Then you can get up and do it again if you want!
I hope this answers your question.

We have acceleration detectors in our inner ear cavities that function as direction-sense mechanisms. By changing the orientation of your head very quickly, you will lose your instinctual sense of which way is up and which way is down, which will cause you to lose your balance.