A flat surface, moving in water, creates resistance. Water resists displacement and penetration by speeding flat objects. Resistance blocks speeding objects on the surface and below. On top of water, blocking is also known as lift. Beneath the surface blocking is called drag or friction.
Water cannot move out from under a speeding surfboard and provides support. The surfboard is no longer supported by buoyancy alone. Speed keeps it from sinking or penetrating the surface deeply. Water cannot be displaced by a speeding board fast enough to allow it to submerge. Thus a surfer can stand up and ride a moving surfboard supported by resistance or lift.
How does this work? Take a flat paddle. Imagine being on a fast moving boat or stand up surfboard, while you stick the flat paddle into the water. Hold it firmly and flat to block water instead of stroking. The board or boat will turn and decelerate. If you are moving fast, you may be thrown off or spun around by resistance. This resistance is hydrodynamic force. When a paddle is suddenly inserted from a speeding vessel, it cannot part or displace water. Water does not compress and produces resistance. This resistance is the same force that produces lift for a surfboard. The surfboard is a wing supported by pressure produced by speed.
The paddle is an appendage. When it is inserted into the water before acceleration, it produces drag. An abrupt interruption or sudden braking is not evident at lower speed. A paddle provides braking and turning with hydrodynamic resistance.
Nose rocker keeps a surfboard from diving under. High nose rocker on a flat bottom surfboard can restrict performance. The underside bend of a nose forms a bow, colored red in the illustration below. When the surfboard points downward, the bent nose is an unnatural appendage. If it strikes water at high speed, the impact can catapult a surfer or stop the surfboard. Like a paddle, nose rocker can interrupt a ride.
The bow is usually out of water when a surfboard planes. The bow must move water out of the board's path when it is not fully planning. At low speed flatness in the bend can move water. As speed increases flatness generates resistance or lift.
Pressure or resistance is produced under a surfboard with speed. The same pressure can push a surfboard back with high sudden impact on the nose rocker bend. The flat bow produces a reverse force or resistance which can negatively affect a rider. In the photo below the board is distorted due to light refraction. Pressure on the nose rocker is indicated by the arrow.
The solution is less rocker with a slight Vee or round bow. A round or Vee surface will ride slightly lower, and push water out of the surfboard's path. The board keeps moving where a flat bow stops. In this phase, a round or Vee bow is faster.