To put it simply, the IMRCs rotate a set of secondly butterfly valves (similar to a throttle butterfly) that open and close the ports to 8 of the 16 intake valves in our engines.
Sucking air through all 16 intake valves is unnecessary at low rpms, so the engines sucks air through only 8 valves. By only having to move air through the volume of 8 ports, this greatly increases air velcoity at low rpms and allows the cylinder to ingest more air at lower rpms. Port velocity is VERY important at low rpms to make low rpm power.
At higher rpms, the IMRC butterflies open to reveal the second set of ports, allowing all 16 intake valves to breath. This is obviously very important, as the primary ports do not have the volume to deliver enough airflow to the engine at higher rpms.
If they become inoperable, stuck in the open position, you will lose low end power. If they are deleted, you will have all 16 valves and ports breathing at all rpms. This will cause a loss of port velocity at low rpms, and cost you low end power.