Bambina seems to be making the common assumption that the Dom is the top and the submissive is the bottom. It may be worth pointing out this need not be the case. BD(DS)SM is a collective term for different insterests.

D - Domination - Dom
S - Submission - Sub
S - Sadism - Top
M - Masochism - Bottom

Although not common it is quite possible for somebody to be both the Dom and the Bottom. I am sure there are many cases of powerful leaders and noblemen who enjoyed being spanked. They do not reliquish the control and authority of the Dom and do tell the top what to do and how to administer the pain.

Although bambina's explanation would in most cases be correct, I personally tend more to sadie's version where the submissive seeks to manipulate (not a direct instruction) the dominant.

Topping from the bottom is often considered as something bad, but it need no be. It depends on the situation and degree. If, in play, the submissive says "I hope you are not going to use the flogger on my ass" this can be seen as a helpful hint to the Dom on what the sub wants and what to do next. But if the sub is trying to take away the decision making and lead from the Dom, that then is of course another matter.