Let's say that difficulty plateaus, that means that mining hardware no longer is deprecating, because output in bitcoins is dependent only on difficulty*hash rate. (It is a common misconception that hashing is a competition like a "race" and others hash rate affects how quickly you can find bitcoins.)

If difficultly is stable then what matters is how efficiently you can mine, i.e. how many bitcoins you can mine relative to how much electricity you spend.

Well if you control say a third of the pool, then you can obviously capture a third of the bitcoins and spend a third of the electricity.

Or, you can mine alternate difficultly periods.

When you mine, the next period the difficulty will be 1. Then you chose not to mine and the pool loses 1/3 of the hashing power, so next round the difficulty will be 2/3. Then it adjusts back and it will be 1 again.

The result is that you get to mine at a lower difficulty than the rest of the population. The difficulty increases from your hashing is paid for by everyone else and not by you, then you get the enjoy the reduced difficulty from others not mining as fast.

This becomes de-stabilising because the rational thing when faced with this is to join you on only mining the easier periods which further makes the problem worse.

This effect starts happening with any amount of hashing power, even with 1% hashing rate you would see a small efficiency improvement by mining alternate difficulty periods.