Inertial Bounce is like making your moves "rubbery." Layers will overextend, then settle into place on position and rotation keyframes. I honestly don't remember where I found this expression, but you can find it in a bunch of different places now.
amp = .05;
The values you want to play with are AMP, FREQ, and DECAY. They are kind of self-explanatory, but when you start playing with them some of them do unexpected things.
AMP (Amplitude): the larger the number the stronger the bounce.
FREQ (Frequency): basically how many bounces you get after the keyframe
DECAY: how long it takes for the bounces to die down.
I use it in all sorts of things, but this works really well to "fake" simulated dynamics. Contrary to typical fashion, if you use this expression, make the out keyframe linear and hard that way the bounce has some inertia from the motion.
EXPLORE MORE with these website that have a few other expressions that simulate motion like this: