The key is to take advantage of the heat quenching ability of cryogenics like N20. Placing the shot right after the screw, where the temps are highest will cause a change of state of the N20 and further heating of the gas. All that latent heat gets soaked up by the N20 and reduces charge temp, just like an intercooler, but better. The extra HP in this case is from increased volumetric efficiency. Basically you are changing the efficiency map of the S/C so it is more efficient at the expense of the nitrous, which is now a gas. However, the nitrous will still decompose and yield O2 so don't lose much and gain quite a bit from charge cooling. The engine will suffer less parasitic loss turning the screw.