basically, you push the button & hold it, then floor it, you're converter will stall (or should) at it's highest limit (no matter how far you push the gas pedal to the floor).
When you're ready to launch, let go of the button!
Away you go.....
If you have a rev limiter, you may not want to totally put it to the floor, depending of course on the chip you're using. It may hit the limiter (which is what its designed to do, but still not real good for your motor). Learn "where" you're rev limiter hits (regardless of what the chip says, sometimes they're not that accurate) compared to where your stall is & shoot for putting the gas pedal just under the limiter but above the stall. Put a slightly bigger chip in if need be to get the most out of the converters stall. (if while you have the transbrake on, & you watch you're tach & slowly give it gas, you can actually SEE what your highest stall is, the rpm's shouldn't go up any higher, at least thats what mine did). As a safety precaution (in case the t-brake malfunctioned), my converter stalled at 4000 so I put a 4200 chip in (on a 2 step, the other chip was a 7800!), so I never "hit" the limiter.
HTH...
