I have heard alot of people using trans brake, but what does it do?

 

When you bump into the starting line and hit the brake, it has a solenoid that actuates against the valvebody. What happens is that the valvebody is tricked into engaging both first and reverse at the same time, locking the transmission up. It allows you to hold the car and add a lot of RPM without trying to use the footbrake. Then when you let go, it's off and running.