Hand planes, Hand boards, Hand guns, Hand surfboard... they go by many names. They are also used in many ways! Hand boards can be made of wood, plastic, foam and fiberglass, inflatables, metal, lunch trays, cork, and many other media. Most commonly they are used on your leading hand when body surfing on a wave. The desired effect is an increase in flotation (planing) and hold onto the wave.
The added benefit of faster paddling with a hand board is extremely beneficial. Especially in critical swim outs or fast take off situations. The boards are also a nice safety cushion for keeping your face away from a reef or rocky bottom in shallow breaks. Although the goal is to never have that happen (Keep your body off the reef! swim out when the tide is right!).
I prefer a small size plane just slightly larger than my hand with downward rails. This way the board is maneuverable during the many types of posture and position changes while riding a wave. The rails should aid you at the drop while your hand is outstretched on the high line. I also like to transition on and off of my back while riding. Or maybe torpedo forward with both hands by my side. Every movement is important, and every style is different. The hand board should be able to go with the flow.
Large hand planes are pretty fun as well. They have a ton of surface area and really propel you down the line. If the board you are using is long enough you can utilize your forearm to add more weight to the board, and in return obtain more lift. I enjoy using large hand boards in two different situations. First, when the waves are too weak to barrel but still set up nice ramps. This is where the larger surface area comes in to help connect sections on the wave with greater speed bursts. Secondly, when waves are fast and hollow. The larger hand board acts like a big wave surfboard and helps to drive your momentum from the drop through the barrel of a lifetime.
The main point of these valuable additions to your ocean quiver is to have fun and catch some waves. If you don't have one, pick one up!