I would be just guessing on it. They could be tuning the traction control system, or modifying the power level itself. During a drag race there's a lot of variables like wheel speed for example. You never want a tire to have 100% traction (called a dead hook) because the jerking will slow you down and usually breaks stuff. You always want a little bit of wheel speed (spinning) but not too much. What that specific limit is on a tesla is, I have no clue. Then you have to adapt that variable to the surface you're on and how your tires are reacting.
There's a ton of things they could be doing with unrestricted access to the computer, but it's just impossible to know for certain.
