A charming man pointed me to this blog entry on a BDSM-themed blog. It's about the concept BDSM people call "topping from the bottom". That's a pejorative term applied to bottoms in a BDSM scene who are too bossy and try to control the top and run the scene.
I admit I have used the phrase in conversation myself, but I think in general it's better to avoid it. It's shorthand, and if you and whoever you're talking to have a very clear understanding of how you're using it in context, it's a useful, if lazy, term.
But it's sort of like the words "bisexual" or "kinky" - both those terms encompass all kinds of attitudes and behaviors, any of which may or may not be true, and if someone doesn't clarify exactly what they mean, a lot of mistaken impressions will result. What does it means to be topping from below? Depends on the top. I myself am not usually bothered by some suggestions, as long as they're offered in an appropriate spirit. Other tops don't want any input once the scene has started.
So it's definitely unprofitable to just bluntly inform someone, "Hey, you're topping from the bottom," as that's both a vague and an uncomplimentary phrase. I advise approaching the matter more diplomatically. "I have an idea! Open your mouth, we're going to use this gag!"