Manifolds are the pipes and ducts that take a fuel/air mixture to the cylinders from a carburetor or fuel control unit (induction manifold) and away from the cylinders to the exhaust (exhaust manifold). The MP guage (measured in inches Hg) measures pressure in the inlet manifolds downstream from the carburetor or fuel control unit.
With a Constant Speed Unit equipped aeroplane the manifold pressure (pressure of the mixture downstream from the carburettor or fuel control unit) is controlled using the throttle, and propeller RPM is controlled using a pitch control.
Not knowing much about helicopters, I would presume a reduction in manifold pressure, and therefore the amount of engine work being done, would bring about a reduction in temperature, although the addition of more fuel might help to cool the cylinders
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Hope this sort of makes sense and helps.
Kermie