how does lean condition bad for the engine??
 

if you have a lean condition and more air than normal... how is that bad for the engine and cause the apex seals to fail?
Seems like running rich would be more dangerous because you have more explosive material inside the chambers

I'm a newb, but from what I gather it's all about the lube and detonation or whatever its called.

Search for "pre-ignition" or "detonation" and read. Basically if you don't have enough fuel it can ignite before the spark is set to fire causing in the piston world you to 'throw a rod' in most cases. In the rotary world you blow an apex seal.

lean rotary = VERY bad!!!
rich rotary = gas guzzler.

running it rich is far better than running it lean. The worst that will happen running rich is you get 10 miles to the gallon and have to clean your plugs every oil change. The worst that can happen if you run it too lean is detonation and after that i hope you got a spare motor lyin' around.

The rough version is this: Gasoline should spontaneously combust in an engine. That means it burns very very fast. When you run lean there is less fuel to ignite and it becomes easier to ignite and will burn faster. Detonation occurs when its so lean that the fuel detonates all at once (instead of a fast burn).

Think of it like a flamethrower, you want it to burn really fast, but you dont want it to explode.