How do I accept a Yield?

I was fighting bandits in the sewers when one of them went into a "cower" animation, and began screaming about how he had given up.

Given that I found his terror endearing, I deigned to spare him. I tried talking to him, to no effect. Then I tried sheathing my weapon - upon which the brigand thought he had a chance, and resumed hostilities with due haste!

Sadly, he did not offer to yield again before I slew him.

And so, the question remained unanswered: do enemies actually surrender, or was this just a clever ploy to catch me with my pants down?

It's a ploy. You have to use your judgement. Honorable people will usually yeild and fall to their knees, if you leave them, they will revert to being a friendly. Bandits, in my experience, haven't.

When you cant kill someone and they go down on their knees they are usually either a quest character or you have gotten their health to just that right amount. if you are attacking a bandit or any other hostile NPC and you leave them they will regain a little health and resume attacking you and you can kill them if they are a quest character you cant, for example, I joined the Stormcloak rebellion and at the end of the questline after you kill Tullius you are supposed to "Go and do as you damn well please, for starters killing any legionares you find in the wilderness" and when i found the camps there is always a "legate" that you cant kill