MSD 2-3 Step
 

What do you need to activate these modules?

Usually just a 12V trigger source. For example, with a 2-step, the first step is active any time the 12V source is not activated. You'd use this for your top end rev limiter. To activate the 2nd rev limit, you use a momentary switch to supply 12V power to the module. When ever you've got the switch depressed, and hence the module is getting the 12V source, the 2nd step becomes active. You'd use this 2nd step for a lower rev limit (most times) for say a burnout limit or stage limit. When you release the momentary switch and the 12V source goes away, the module jumps back to the 1st stage limit.

I use the 2-step to set my launch RPM and have it tied to my transbrake switch. When I'm in the beams and on the transbrake, my engine revs are limited just below the stall point of my converter. When I come off the brake, the converter flashes for a little extra ooommmphh out of the hole.

Thanks.