constant speed prop question

[capt. skidmark]

dear fellow instructors,

i have had a question from a student why exactly the manifold pressure increases when the prop lever is pulled backwards and vice versa.

i tried to approach this answer with the total pressure formula etc.
or that the pressure builds up since the prop gives more resistance to the engine when the prop is in high pitch, but i started to doubt if this is correct.

i cant find anything in my books about this, can any of you give a simple (american style) answer?

thanks a lot,

sm