As a wave approaches, you slide back on the mat and sink the tail as deep underwater as you can manage. When the wave gets to you, you slide forward and allow the buoyancy of the tail emerging out of the water to give you a nice little energy surge. Often times you'll catch a wave that might otherwise pass you by. (Tossing in a giddy grin, as demonstrated by the mat rider above, also helps!)
George Greenough executes a subtle version of this technique...
