Bees, like most insects, don’t have muscles directly connected to their wings. Instead, the flight muscles are attached to the roof wall of the thorax.
When it is pulled down, the thorax suddenly pops inward, like the dent that appears when you squash a ping pong ball. This flips the wings upward.
A second set of muscles squashes the thorax lengthwise and the roof pops up again. This happens about 190 times a second in honeybees and the rapid clicking of the thorax wall sounds like a buzz to our ears.
Bumblebees also use this feature to shake pollen out of flowers.