What is "flexes"

Usage examples of "flexes".

The man seems poised to leap away if the tall apparition flexes so much as a finger blade.

The cord flexes, stretching the living skin of her neck out for just a moment, and then relaxes, leaving nothing but smoothness and calm.

He flexes the deck of cards in one hand and shoots them across to the other.

The stairway flexes and recoils under his weight, reminding him of the jet's fragility.