What is it called when you have the rear tires spinning while the car is stopped, right before a race starts?

Burnout - it warms up the tires.

You can accomplish the same result by putting a line-lock on the front brake lines - you press the brake pedal and hit a button that closes a valve that keeps the hydraulic pressure.

Since the rear brakes aren't usually doing much of the braking anyway, most just choose to put their foot on the brakes and hit the gas. Don't do this at home, though......

Its called a burn out and not only is it cool to cover your friends in tire smoke, it also heats up the tires making them sticky. This stickiness results in more grip which helps launch the car

it's called a burnout. just hold your brake down and hit the gas. if you want to do it really cool pour some bleach under the tires it will smoke alot

You don't...

The "burn-out" is to warm & clean the tires but

it is not done at the starting line...