TL;DR: The idea is to move all your tokens to a random split in the last possible block within the 1 week debating period. The attacker could only guess.

PROBLEM: You are right, there is no guarantee that miners will mine your transaction at the last block. This is a rather unelegant solution.

How does a split work:

create a proposal with newCurator=true , recipient=<new curator address> You (and the attacker) vote in favor of the proposal At least 1 week debating period in which a token holder can call splitDAO

See How to Split the DAO

So what Christopher Jentzsch is proposing is that with enough accounts and enough proposed splits, you can move your tokens in the last block before debating period has ended. This is not guaranteed as the miner can not be coerced to mine your transaction right in that block.