Manifolds are just a set of pipes that lead into "Induction" and out of "Exhaust" the engines cylinders.
The power an engine can produce is related to the RPM and manifold pressure.
The faster the RPM the more bangs per minute and therefore more power.
The induction manifold pressure is related to the amount of "charge" i.e. fuel/and air that can be stuffed in to the engine thus giving a BIGGER (more powerful) bang.
When the engine RPM is constrained i.e. Constant speed propeller or RPM contol of the main rotor the varying element of power is the Induction Manifold Pressure.
That's my understanding anyway.