To everything there is a season,
and a time for every purpose under Heaven.
We move about in cycles. Left/right, forward/back, up/down, expand/contract, extend/retract, exert/relax - those examples are common and easy to see. But there are other cycles we probably think less often about. Important cycles like balance/unbalance and strong/weak.
If I understand what I've read and heard about the old Kito School of jujitsu (from which judo and aikido were partly derived), one of the central theories of Kito was that our ki - our energy and potency and strength and vitality - was constantly ebbing and flowing. Thus the name of the school ("Kito" = "rise and fall"). This idea manifests itself in techniques in which tori synchronizes with uke such that the decisive encounter takes place when tori is strongest and uke is weakest.
This Kito principle (as I call it) is central to aikido and judo.
We move through these cycles at a certain rate based primarily on our body size. It takes some finite amount of time to go from strong to weak, and back to strong. Trying to speed up or slow down the period of these cycles is counter-productive, so you need to learn to recognize these cycles and go with the flow. Relax and go along for the ride until you are stronger and the opponent is weaker.
Sure, you might get beat up while you are in the wrong part of the cycle, or if your synch with the opponent is non-optimal. But tensing up and standing still and fighting the cycle won't help either. You'll still get beat up, but you'll be tired too!
We don't make the waves,
We just ride them.
[Photo courtesy of Nathan in SanDiego]