Ah, I just wrote my essay for my promotion test about this!
For me, it is about recognizing which moments in a technique require speed and power and which ones don't. For, say, a front snap kick - well, that's a giveaway, right? It has the word snap in it! Lifting my leg to the chamber doesn't require much power/tension and can be done in a relaxed (I wouldn't use the word loose, myself) manner. Even the very beginning of extending the leg doesn't. It's that last moment - the snap - that requires the speed and power. I call the more relaxed portion of the technique "toned alertness" - kind of an awkward phrase but I think it's correct. I titled my essay, "Relaxation, power, and the conservation of energy".
I practice each technique thinking about the stance, balance, movement and what portion of the movement and my body can be relaxed and what portion more powerful.
Hope that helps some.