Shp is a measure of the hp at the output shaft that the proppeler or rotors are attached to, not the output shaft of the engine(s).
This way it takes into account having two engine hooked together (as in the chinhook) or changes in torque through a gearbox, (most small planes use some kind of reduction drive).
Its usefull for knowing what size and pitch of propellar/rotor can be used, and how well it will preform.