The forces on the spherical cow are as follows:

Black Arrow: Weight of the cow due to gravity = MG

Blue Arrow: Buoyancy force acting perpendicular to the plane of the surfboard.
Buoyancy is defined as the magnitude of the weight of the fluid displaced causing the cow on the surfboard to float

Red Arrow: The resulting Force Vector pushing the cow forward through the wave

To accelerate down a wave (F=ma) you can move your body from a couched position to an upright postion.
By straitening your front leg you can push downward with a force greater than that of gravity and therefore accelerate.

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