[Bicycle Bob]

Try imagining that you are on a space walk, and have a heavy weight on one hand. If you push it away, your combined center of gravity stays in the same place, so you move away from it also. If you are dealing with a satellite that weighs the same as you, both objects would move the same amount. If you grab the space station, you do most of the moving, but it still shifts.
So when you raised your hand in class, the earth moved away from your hand a tiny amount, and also slowed it's rotation due to increased moment of inertia. The theory is easier than the physical measurement of the extreme example, though.

J.E. Gordon has some choice paragraphs on how floors push up on your feet in "Structures." Also a cartoon, captioned "Do stop pulling on kitty's tail, darling!" "I'm not pulling, mummy, kitty's pulling!"