Complex sentence: The horizontal area in the tunnel fin also acts as a means of transferring rider energy into forwards thrust through board flex... basically it's like a dolphin tail... the board pumps up and down and drives the' fluke' at the tail.
Split into simpler sentences: The horizontal area in the tunnel fin also acts as a means of transferring rider energy into forward thrusts through board flex (similar to a dolphin tail). As the board pumps up and down it drives the'fluke' at the tail.