This age old technique assures the sharpest possible image when using any lens that attaches to a tripod via its tripod collar. You rest your hand on the lens just like you'd rest it in your lap.
Vibrations are what causes images to be out of focus, vibrations that normally start at the camera.
These vibrations travel as a wave to the front element and if they are not stopped, they travel back through the lens to the film plane, causing images to be out of focus. The simple resting of a hand on the lens barrel stops this wave before it reaches the front. No, a bean bag resting in the place of the hand is not a good idea for many reasons. This again is another way to minimize or eliminate any vibration that might start with the taking of the photograph. If you can't remember to press against the eyecup like I did in the beginning, put a sticky note on the back of the camera to remind you.