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There are two methods for diving under a wave; the first known as a DUCK DIVE which is used for short boards and it involves pushing the tip of your board under the water while lifting one foot high into the air. The torque caused by this action will submerge the board and surfer under the bulk of the wave to prevent the forces from pushing the surfer back to shore.

The second method called the TURTLE ROLL is used primarily for
long boards and is executed by rolling upside down with the board
between the surfer and the wave. This is to minimize the friction
caused by the wave by using the board's smooth bottom edge.
This method is not as effective as the duck dive, as the wave will
push the surfer back toward shore, just with not as much force.