Informed consent is what makes an activity power exchange instead of abuse... and, fundamentally, what makes power exchange fun.
Consent applies just as much for tops and dominants as it does for bottoms and submissives.
For example, if a woman asks me to tie her up, I am responsible her safety for as long as she is tied up. I need to stay with her so that I can untie her in case of an emergency, to ensure that the ropes aren't cutting off circulation, and so on. When I reply “yes” to a request to tie someone up, I am consenting to taking on that responsibility.
Informed consent is subtler but still vital. Informed consent means everyone fully understands what is being consented to.
For example, if someone doesn’t disclose that they’re prone to seizures when they ask me to tie them up, it’s not informed consent for me to say “yes”. I haven’t been given an opportunity to understand the ramifications of the responsibility I’m agreeing to.
Nice crisp clear informed consent where everyone fully knows what's going to happen and has enthusiastically agreed to it is what makes power exchange fun.
See also Limits and Surrender.
Read more about Power Exchange.