There's something about whispers that are so intimate and immersive...

Say my name. No, seriously. Say it. You'll find it brings you pleasure, a warm fuzzy feeling that is more then a little bit sexual... "Submit. Speak. Cum." is about getting the listeners to say the trigger words themselves, as a bit of an added layer of obedience to me. The HFO aspect of it is also implied, rather than an explicit command. There's nothing stopping somebody from grabbing on and going nuts.