The approach this tool takes is that the first transaction has a fee small enough that it won't propagate quickly. But it might reach your zeroconf-accepting counterparty (e.g., Shapeshift.io) before it reaches a miner. Then once your payment has been accepted you broadcast the double spend with the higher fee in the hope that the second transaction is the one that the miner of the next block happened to see first.

So it is a race attack that takes advantage of the propagation latency that occurs with transactions paying only a really low fee.

Thus even though Bitcoin Core nodes won't relay your transaction that has a double spend, some nodes that you broadcast the second transaction to might not even know about the first one and will relay it.

Also, keep in mind that BitcoinXT nodes will relay double spends. So even if BitcoinXT doesn't cause a fork, enough XT nodes out there can still make double spending like this easier.

It's really a moot point -- almost nobody accepts zeroconf from an untrusted party and those that do (and aren't double spent into insolvency) are using scoring that would ignore a transaction with such a low fee. Shapeshift.io uses BlockCypher's scoring plus some other custom protective measures, according to this: http://reddit.com/r/Bitcoin/comments/387y84/from_zero_to_hero_bitcoin_transactions_in_8/crt098j

As far as BIP101, if that causes a fork you wouldn't do a race attack like this. You would simply taint your pre-fork coins in BitcoinXT with newly mined coin, and then spend (to a new address) from there, while also retaining the ability to spend the pre-fork coins in Bitcoin Core at your leisure.