Ruven said,

>>> I am not sure of what a power exchange is. <<<

Using your experiences, as you have described them, imagine a submissive coming to you and begging you to do sadistic things to her in a situation where she has no safeword, no way out of the scene until you decide you're finished. You have all of the control, period, including control over how long the encounter lasts. A situation like that involves mild (because short-term and temporary--it ends when you end the scene) power exchange.

I have a hard time seeing a scene where a submissive has a safeword as a power exchange, because the submissive isn't really ever helpless: the scene could end at any minute at her whim, not your whim. The submissive has the ultimate veto--the ultimate control.

I'm in a long-term master-slave relationship, and I live with no power over anything at any time, the scene never ends, and it's made me rather spoiled, I'm afraid. I so love the thrill of helplessness that I would find a situation in which I had to use a safeword boring as heck (and I'd probably use it way too early--I'm something of a wimp when given a choice--g).

Non-safeword scenes can be frustrating in a different way: your top or dominant may end them long before you want them to! One evening my master was caning me rather rapidly (about 200 strokes over a 30-minute period). I was really into it, and so it surprised me when he suddenly stopped. I asked him why he did this, as part of a more-general 'plaint of "Why don't you ever take me past my pain limits?" and he replied that this time I had started to get shocky and that he always stops when that happens. There is a small medical risk involved with shock that he simply doesn't want to take with me. Anyway, although it was frustrating for me, I still liked not having any control over the scene's ending; I liked being able to trust him to end it as he saw fit.

Getting back to the topic, I think power exchange occurs on a continuum, but I think that the mildest end of that continuum occurs when the sub gives up control completely for a short (but unpredictable and uncontrollable) period of time. If you have a safeword, you still have control, even if you choose not to utter it. Hell, that choice itself is control.