Let’s take a plunge down a very steep wave. As you struggle to keep the nose up, you lose control. The surfboard free falls to the bottom of the wave and the nose is buried in the dark depths of the sea. The surfboard stops moving abruptly and you are catapulted, flying through the air weightless. You are hoping the surfboard doesn't hit you as regretful thoughts rush through your head. “I know, I should’ve gone to church. Oh no, I didn’t hug the family.” You impact water with stinging pain. This is accompanied by a jolt to the head that compresses your neck, sending numbness down your spine. The wave breaks on you and tosses you around as your lungs crave oxygen. At last you surface gasping for air. You don't think nor care about why the surfboard suddenly stopped moving. You just assume it stopped 20,000 leagues under. The belief that surfboard stops because it submerges is only partially true. Water over the top of a surfboard may add a little resistance, but; most resistance or drag is produced under the board by a bow wave. A bow wave is generated under the board by impact on the surface and underwater.
Nose rocker puts a bend in the surfboard’s underside called a bow. The bow is a landing pad where the board impacts water. Most modern bows are flat. A flat bow displaces water slowly causing a speeding board to stop in its tracks. On impact, the flat bow pushes water forward then directs it around the board. This process takes too long for a speeding surfboard to continue movement. Modern flat bowed boards with high rocker can recover at lower speeds in smaller waves. A surfer formerly progressed through gradual acceleration with moderate fluctuation in speed. This is no longer the case in modern surfing with aerial flight. Aerial maneuvers and free falls have changed surfboard performance. A surfboard frequently goes from high speed to an abrupt stop. A skilled rider cannot remain standing with extreme fluctuations of speed.
The surfer's weight is a key factor driving a nose poke. Caio Ibelli gets off his board when the nose pokes. Removing weight allows the board to move, in the Quiksilver Pro France.
Surfing is a gravity sport. The surfer's weight drives the board. In a nose poke, weight drives the board into water creating resistance or friction, represented by "f" in the diagram. The diagram is an inclined plane which illustrate gravity forces. Friction is wave making resistance or a bow wave, that forms under the board, sometimes underwater.
In the next photo Caio's board appears distorted or broken because it is under water. Note the bow wave produces spray which engulfs the nose even when it is submerged below the surface. This spray shoots over the board and does not produce significant pressure to stop movement. The board decelerates due to the surfer's weight which creates drag under the board at the bow, in the form of a bow wave.
In the final photo, Caio lays back on the wave and unweights the surfboard. The nose of the surfboard resurfaces. This proves that the surfer's weight not water drives a nose poke underwater. Caio almost gets back to his feet, but gets engulfed by the wave.
Surfers like John John and JOB occasionally use a similar tactic in a rail grab drop, at spots like Pipeline. They fall and take impact on their back and butt to keep the bow from stopping movement. Their body absorbs much of the impact in a free fall.
It takes special skill to successfully get weight off a surfboard and complete a ride. An alternative to an unweighted landing is to combine moderate rocker with a softer rounder or vee bow which cushions landing and pushes water out of the rider's path.
Surfboards with alternative bows move by parting water. These bows continue moving where flat bottoms stop. Surfboards capable of moving while partially submerged can prevent stalling at high speed. A flat bottom surfboard is not efficient at moving below the surface, it is designed to ride on top. This makes it vulnerable to stopping in a wave trough, in chop, and in a flat aerial landing at high speed. A compromise in design is a semi displacement hull which features a planning aft section and forward displacement shape. I am testing shapes which ride out of shallow pearls.
Photos and video from Youtube.com, WSL Heat Analyzer Quiksilver Pro France