The gong, or tam-tam, is a large circular metal plate, usually suspended from a frame. The metal plate is struck with a padded mallet which vibrates and vibrates and vibrates.
The player gently taps the mallet a few times towards the center of the plate to “warm” the instrument and to get the vibrations going. When the gong is cued in the music, the player will use the mallet to make a hearty strike which really gets the vibrations going, and therefore, creates a unique sound that almost sounds like a wave crashing against the shore.