Note that the engine in Mad Maxes vehicle never did have a functional supercharger on it. That was a hollow cavity with electric motors in it that gave the appearance that it actually did something.
I think where some of the confusion here comes from the idea that the Supercharger itself is compressing air. In point of fact, the M90 Eaton supercharger doesn't compress the air as it moves through the supercharger. The M90 is a roots style positive displacement air pump.
For each revolution of the rotor pack, a fixed volume of air is forced through it. As long as there is nothing preventing this air from moving, the air doesn't become compressed. As long as the air isn't being compressed, it doesn't impart any negative force against the rotors.
Thus the bypass valve allows any excess air (air not being pulled into the engine) moved by the supercharger to be recycled into the input side of the supercharger. Thus when in bypass, the supercharger will effectively just rotate the same air out, then in, then out, then in and so on until the engine uses it up.
When the bypass closes, then air moved by the SC has no where to go but along the path through the IC piping and into the intake manifold. Once in the intake manifold it can either enter the engine through the valves during combustion, or stack up there. Because the Supercharger moves a greater volume of air through the intake tract than the engine can consume it, positive pressure is developed which you see as "boost".
It is this positive pressure in the intake tract that fights the further rotation of the rotor pack as the rotor pack attempts to insert more air into the intake tract. This is what causes the supercharger to consume engine energy from the crankshaft as it fights this pressure that develops in the intake tract.