Well it's basically the submissive giving orders. For example, when it comes to spanking, the spanker is supposed to be the one in charge. Gaging how hard to hit. Where to hit. When to hit. And with what to hit. But let's say the sub called the shots on all of that.

"Slap my ass hard. No thats too hard. You know what, I changed my mind, use the brush instead. Yeah that's right. Now tell me I'm a dirty girl. You're hitting me too hard again. I said stop that. Okay now kiss me and hold me tight."

And all the while the Dom is going "Yes Dear. Yes Dear. Yes Dear."

Who's really in charge here? The point of submission is to give up power to someone else so that they lose control over what happens to them in intimate situations. But, when a sub 'Tops from the Bottom', there is virtually no power exchange. The Dom is following all of the subs rules without pushing any limits or adding his own flare in anything. Almost a reversal of roles except the sub is still on the receiving end.