Joel - It depends on a lot of factors and the sophistication of the electronics used
. Having said that, the easiest and most common approach is to vary the slew rate of engine acceleration (ie: RPM/sec) vs. the ignition timing
. The amount of retarding the timing of course varies with the tune up and the application. Perhaps overly simplified, but that is the gist of the control methodology. Another and the most generally applied is to manually vary the throttle position. Some drivers have the knack and some don't. Others depend on more complex electronic controls. As in most things, applications, opinions and results vary.
Regards,
HB2