because

Interesting class -- I showed up an hour late (dealing with end of 2Q stuff at work) and it was a hair short of sweltering in the dojo, but we dove right in.
Started with shin as always, and everyone looked pretty good. Spoke a bit about getting some more extension without necessarily stepping/sliding forward, that is getting maximum reach without closing the distance too much.
Then we did sa, and everyone looked really good. The key is to stay fluid, and keep the hips moving forward (left) without pulling them back and then moving in again. Very hard to keep the draw and cut one smooth motion, but it feels really good when it works.
Finally we went through yochuin. First and most important: do NOT look at the foot you may or may not be cutting! Watch the person onto whom the foot is attached, and get off the centerline; open up the left side of the body, retract the left shoulder and left knee and extend flat -- tsuka flush with the arm and a straight extention of it. Reach deep and force him back, then follow him in and finish him. A simple kata, but quite involved and as always many details easily missed.
Most important is to simply do the kata as it is intended. First believe that the kata "works"; kata exist in a made-up world of marginal reality, where the enemy moves in a specific way, to which you react in a set way, and you win. Later on you get to play the "what if" game -- what if he does this and not that? What if he's standing? What if I did this first? But for now, we just do the kata, and instead of second-guessing it -- this is easier, this feels better, why wouldn't I do this? -- we just do it and try to figure out why it works, how it works.
There is nothing wrong with the kata; there is everything wrong with us.