The above link takes you to a fascinating and spirited discussion on Sanchin back in 2001.
I've picked out a few quotes from various people related to a term that refers to what I believe is a very important concept in Uechi.
...Chinkuchi is the cultivation of energy (ki) through proper body mechanics. This is done with the tightness and the breathing in San-chin....
...My sensei uses the word "chinkuchi" when refering to the natural "locked" body katachi or postures. It is the point at which an externed arm, stance, etc...is in the position where one attains the most strength or power with the least effort. For example tsuki chinkuki is -horizontal to the ground, shoulder blade pushed back, lats locked down and shoulders rolled forward. this is not to say there aren't other ways to execute a tsuki, but that this position in neither more easily pulled nor pushed, raised nor lowered. This is the best "return on investment" as it were for this technique. This is one of the keys to sanchin, it teaches posture, body locking with minimal effort, and still being mobile....
Sanchin...it is done in two ways, with tension and with speed. ...This is done for health and for the cultivation of Chinkuchi. Sanchin is prolonged Chinkuchi.
Sanchin is prolonged Chinkuchi.
I know for some folks that isn't going to mean much - but for me and the way I understand my training - that is a pretty darn good way to describe a key concept of sanchin in a nutshell.