The finger is three bones. On a press the base knuckle bends first, then the middle, then the tip; each bone shortens by the cosine of its angle as it tips toward the screen, and goes a shade darker. The ring opens when the fingertip touches. On release it springs back past straight and settles.
Six times the real size, tapping every 1.8 seconds.
Watch the joints bend in turn, then the spring back past straight.