The simplest way I have ever heard an internal combustion (reciprocating engine) described is as being basically a simple air pump.
That is imagine you have an infinite amount of power available from say an electric motor attached to the crankshaft of your engine spark off, fuel off.
The effort required to turn the engine will be a result of the amount of throttle applied.
That is close the throttle/butterfly and restrict the air flow when the engine is turning (that electric motor) and it sucks, big time, (reduction in MP), fully open the throttle, then the MP should now approximate ambient pressure except the mechanical restriction of the gas path going round corners, friction and inefficiencies effectively reduces the presssure to around 2" less than ambient.
See where the term throttle comes from. Throttling the engine is like choking someone by you know how!
Therefore when the engine is at rest, throttle (butterfly) open or closed (engine not turning) the MP should approximate ambient pressure.
Take off power not more than around 2" less than ambient.
A good way to check whether the correct power is being produced. More than 2" then there is an industion problem and whilst you may be getting RPM you will NOT be getting max available power.
See the fence/trees coming.
Ok so our electric power bill is getting a bit high now and we have run out of coins to feed the meter.
So we add fuel and a spark every now and then to make it self sustaining.
Same principle applies we control the amount of air available to pass through the engine and therefore the power.
Think it through.
Capiche.