The Libet experiments and what they mean for how the brain initiates actions

I'm wondering if anyone has some further thoughts on this topic as regards a particular aspect of the experiments and data. I've read several more papers on this but something that isn't clear to me is how Libet (and others) have been able to determine the timings according to the subjects own introspective assessments.

The times that were recorded were W, M, S and 0, where W is the time the subject perceived the urge to act, M is the time that the act was perceived to occur, S was a time that the subject perceived an electronic stimulus of the skin and 0 was the time of the act according to ECG traces.

Subjects were to allow the act (a wrist or finger flexion) to simply occur - that is, to be voluntary and not in response to any external stimulus - and not to prepare or pre-plan the act. Now, if I try this myself, I am simply not aware of any 'urge to act'. I can just sit here and then have my finger flex so I can clearly identify the moment it happens, but an 'urge to act'? Not at all. Even if I try to keep my mind 'clear', I am simply not aware of anything other than the movement.

Does anyone have any thoughts on what exactly Libet's subjects might be reporting? Especially when you think it's within 200ms of the act occurring, that's a level of fine temporal distinction I don't think I could do.