It's all about the triggers-- you have to find someone's triggers, and then you can control the triggers to control the act. For "cumming on command," it is mostly about altering the trigger-- a Dominant changes what makes the submissive cum.

"Cumming on command" varies as well... sometimes it works with just a phrase, sometimes more (i.e. touch to a specific place +/- a phrase) may be required. It is, as always, varied from submissive to submissive.