This is a Glitch in GTA: San Andreas that allows CJ to fall from any height and survive.
How To Use
The only step to using the glitch is to run, not jump, off of a building. To fall from a vehicle high in the air and survive, build up speed and then exit the vehicle. Do not release the exit button until you have landed safely. Both of these methods will only work if CJ has full health, and will deplete him to a very low health level when he lands.
It is currently unknown if this glitch exists in the Xbox, PC, or Greatest Hits versions of the game.