Gutting the exhaust system and letting it free-flow means the exhaust stroke takes less energy, so the engine has more umph left over to suck in air.
Freeing the air intake system also means the intake stroke of the piston takes less energy, so the engine has more umph to suck in air.
In either/both cases, the engine pistons are working less to suck in air or push out exhaust, so it runs at a higher rpm for the same throttle plate position.
I've said many times that airflow into an engine is very complex, and I don't profess to understand it completely.
To begin with, it is not a smooth stream of air, but a pulsating flow. 3/4 of the time there is no air going into the engine cylinder, and then there's a quick gulp through the carb. The other cylinder on a Rebel takes a gulp on the next engine revolution so that air enters or exhaust leaves the engine about half the time in quick bursts.
The stop and start nature of the flow is subject both to the static restrictions in the gas paths and also to dynamic wave action reflected in the intake and exhaust tracts. Ideally, a reflected low pressure wave arrives at the exhaust port just as the exhaust stroke is finishing, resulting in a thorough scavenging of the cylinder making the maximum room available for the intake stroke to refill the cylinder with fuel and air.
The cylinder scavenging and filling efficiency is also very complex and depends a lot on the dynamic reflected exhaust waves from the previous exhaust explosion helping or hindering instantaneous flow at various RPMS. This is why cutting exhausts and modifying intakes has significant operational effects that may not all be positive.
The pistons are the driving force for this flow, but their effect is constant, based only on engine RPMS . The intake/exhaust resistance to flow, both static and dynamic, governs the net flow, not the "umph" of the engine.
The fact is, the more fuel/air mix that can be stuffed into the cylinder at a given rpm the more "umph" or power output will be available at the crankshaft. Typically a given setup will result in optimum flow at specific RPM ranges, and engines can be tuned to work best at mid range RPMS (like the Rebel) or at high RPMS (like sportbikes).