Mainly by the strength of the big arm, chest and abdomen. At the same time, the big arm exerts force inward, and the chest exerts force outward, and then the process of exerting force gradually decreases, so that you can make a chest explosion, which is what you call shaking your chest.

This is a kind of chest pop, and the other is the big arm force, but the abdomen and chest force are in opposite directions. This is more difficult than the first one. I suggest practicing the first one first.