It is based around the 90% forger getting penalised if he doesn't forge a block in turn. This is detected by the remaining 10% and the 90% has forging power reduced to 0. The 10% then effectively go to 100% of forgers and the dance continues.
This implies that even if the adversry skips forging only 11.1% of his blocks, he penalised so much, that his mining power reduces at least 9 times.Edit: My last sentence is wrong. Having 90% of the coins, the adversary does not need to skip forging even a single block. Just orphan all their blocks, that's it.
