4 cycling a 2 cycle engine?

I had a chance to go out to Whittier Narrows and watch some people who knew how to fly pratice there patterns.
I new to this stuff and don't know a whole lot about it, but these guys were talking about there engine "slipping into 2 cycle" at the wrong times during there flight.
What are they doing? I did notice they seemed to be running very rich. Is there a tuning trick to this?
Thanks,
Rob Johnson
Orange Ca.

The basic idea is that you run the engine rich, it will 4-cycle, bang every other revolution and give you about 75% of the power. When you put a load on it, as in a maneuver, it will go into a 2-cycle and give you more power when you need it. The problem is to get the timing, compression ratio, venturi size, fuel, prop, and phase of the moon right so that it will do this as wanted. Now we will let the experts expound and explain.