I've been thinking a bit about the timing of events in the first two techniques of Junanahon kata. The way that I teach these techniques, they start off with the same actions - as uke invades ma-ai, tori gets his hands up between his face and uke's, and tori also steps off the line of attack toward the inside. But tori ends up in different places with respect to uke in these two techniques. In shomenate, tori ends up inside uke's arm and in aigamaeate, tori ends up outside uke's arm.
Same actions place tori in two different places. Sorta like Jeff Goldblum in Jurassic Park lecturing whats-her-name about chaos theory - microscopic differences cause the water to run off of her hand in different directions.
But it turns out that it's not just how uke and tori happen to knock together that drives these two techniques. The timing of tori's actions tends to place him either in front of or behind uke.
Tori's step out of the way has a wave like, down up quality. As a general rule, Tori wants to synch the rise and fall of his arms with the rise and fall of his body so that he is not raising his arms as he is dropping out of the way. This means that he can either raise his hands then drop out of the way, or he can drop out of the way then raise his hands.
If Tori raises his hands then moves, he tends to end up behind uke's arm - aigamaeate If he drops out of the way, then raises his hands, he tends to end up inside uke's arm - shomenate.
Photo courtesy of Nicolas B