Why does manifold pressure increase with power?
Manifold pressure is actually a measure of the vacuum pressure between the throttle and the cylinders. The more the throttle is opened, the closer that manifold pressure returns to atmospheric pressure.  

The amount of fuel provided to the engine depends on the pressure difference between the manifold and the throttle body.  The throttle body pressure will decrease as the throttle is increased, because of greater rate airflow across the venturi (faster air -> lower pressure).  The manifold pressure will increase as throttle is increased, due to the greater flow of fuel/air mix into the manifold (greater mass of air flowing into a fixed volume -> higher pressure).  So the pressure difference will get larger as the throttle is increased.  

This link has a pretty good description of what's going on.