Action is the mechanism that enables a key to sound the pipes. Tracker action is where the link between the key and the pipes is entirely mechanical - basically, when you press a key a succession of rods and sticks pulls down the pallet under the pipe to let it speak. There's a diagram about halfway down this (rather technical) page that shows how tracker action works. http://www.pykett.or...gan_actions.htm
Ooh I've just started dipping into that page. I think it might be of great interest to my son at some point. We do run rather in the maths and physics direction in our house!