There's no sudden light-switch that goes off. It's all a linear effect based upon physics:

power = (force * distance) / time
power = PedalForce * (distance/time)
power = PedalForce * RPM

What happens is that there is limited force that you can apply to the pedals based upon your leg muscle's contractile strength (roughly related to number of recruited fibres and their cross-sectional size). At some point, you are pushing on the pedals at 100% and can not push harder. Yet, the power-generated also involves RPM as well. If we compare two scenarios of identical power-outputs, but at different RPMs (60 vs 120rpm), we get:

Power1 = PedalForce1 * RPM1
Power2 = PedalForce2 * RPM2
Power1 = Power2

RPM1 = 60
RPM2 = 120
RPM1 = RPM2/2

PedalForce1 * RPM1 = PedalForce2 * RPM2
PedalForce1 * RPM2/2 = PedalForce2 * RPM2
PedalForce1 = 2* PedalForce2
PedalForce2 = 1/2*PedalForce1

What this gives us is that spinning at 120rpms yields the SAME power (and same speed) as pushing twice as hard at 60rpms. When you are pushing twice as hard, your muscles are not as efficient (ATP generated per O2 & glucose consumed), and they will fatigue much, much faster. When you're doing a 100-mile ride at 20mph, the person who's pushing 60rpms will go just as fast as the person spinning 120rpms. However, the 60rpm person will wear out their muscles, fatigue and cramp up faster and most likely blow up and drop out sooner. In general, spinning faster allows you to generate the same or MORE power while pushing on the pedals less.

Another way to look at this is when you're already pushing 100% on your pedals and can't possibly push any harder. Two sprinters are of equal strength and can push on their pedals with 250-lbs of force. However, one pushes with all his might at 60rpms while the other pushes just as hard, but at 120rpms. The 2nd sprinter will generate DOUBLE the power and go 26% faster with exactly the same strength!

As for "optimum RPM" , there isn't one. There may be a different one for each person. That is based upon technique and form. Different people will lag their upstroke/dead leg more than others, thus requiring different amounts of wasted down-force from the other leg. This imbalance will cause rocking and bounding and limit how fast you can spin. Some people are better at applying force 90-degrees to the crank all the way around the pedal-stroke than others who may be pedaling squares. Riding one-legged on a CompuTrainer can generate a polar plot and show you exactly how smooth or not you are.

Once you get the spinning down, it becomes a delicate balancing act between your muscular and cardiovascular system:

Spin Faster = tax the muscles less, tax the heart/lungs more
Spin Slower = tax the muscles more, tax the heart/lungs less

This is why bulky sprinter types would ride at lower-RPMs than thin climber types who would be most efficient spinning. You can figure out the balancing act with a HRM.

Also the speed at which you ride will dictate the most effective RPM as well. Low-speeds of 15-20mph would favour lower-RPMs (70-90) because it doesn't require extreme amounts of pedaling-force and lower-RPMs will keep your HR low as well. Higher-speeds like 25-30mph would need higher-RPMs 90-110 so you can tax the legs at the same intensity as the heart & lungs (and blow them up at the same time). All-out sprints require the highest RPMs of 130-150rpms in order to generate the highest power-outputs and obtain the highest top-speeds possible. Of course, it takes some training to effectively spin the pedals and apply force at those RPMs. If you can spin 200rpms+, then you'll most likely be efficient at 150rpms. It can take a decade of training to reach this point.