I think this is really a just communication thing: I’m with someone that says “do what you want”, and I’ll say it myself too, but we’ve been together long enough that there’s an implicit understanding of what that phrase means.

The point is that it’s a turn on to feel desired, even more so when you can just “give up control” and they’ll still do something you like. My experience is that the whole “do what you want” thing isn’t something you say to turn on your partner so much as yourself. Specifically, when I say it, it’s a way of communicating what I want (because of the mutual understanding of what it means) while keeping the pretense that I’m giving up all control (which turns me on). Likewise, when my partner says it, I know what she wants me to do, but keeping the pretense that I’m in complete control is a turn on for both of us.

Of course, this kind of mutual understanding has been built up over time, and I wouldn’t recommend communicating like this in a fresh relationship or one night stand.