Attuned
It is hard to describe, but I am gonna give it a shot. First of all, I am not a pure 'lee, and I am most of the time on the giving hand. However when we switch, the thing I like the most about my partner is the extremely empathic way in which she tickles me. She can be harsh, merciless even, but she will never ever do something that would make the experience turn unpleasant to me. She knows exactly what to do as well as when and how, without having to ever communicate with me. Like she really can feel what I feel, and adjust instantly, making me howl when she feels she can push me and changing spots swiftly when the sensation starts to feel less intense. Likewise, we never ever have to use a safe word, because she always, always stops at the very moment that I am starting to feel I can't get anymore of it, again without having to tell.
To me, it is a little bit like a musician can feel the pace of a given piece and adjust accordingly (I watched Whiplash recently ^^). I have always been a switch, but I never liked it so much than with someone who can be like that.