Notes on the Tai Chi Training Method called Ti Fang By Stephen J. Goodson
"The effect of his intent.'' --Chaucer.
You cannot follow something and apply force to it at the same time without effecting the speed or direction of the thing you are trying to follow.
So, when practicing the two-man exercises of Tai Chi Chuan be aware that there can be too much pressure, which changes the direction or speed of our partner. There can be too little pressure, where you can't hear their movements and are constantly trying to catch-up with them. And there can be just the right amount of pressure, where you are connected as if part of them, so their movements move you, but you do not effect them. This is following.
This "connected as if part of them" is referred to as "four ounces" and is just about 4oz. of pressure.