Easy. Send a tx to yourself, use those inputs for a second transaction to someone else who accepts 0/unconfirmed transactions, then mutate the txid of the first transaction, making the 2nd transaction invalid, allowing you to double spend the 2nd transaction.
You could do a double-spending without this attack This attack doesn't help in this...
It makes it much easier. More nodes will accept the mutated transaction then would accept a transaction that outright conflicts with an existing transaction in it's mempool so the chances of getting the mutated transaction confirmed are greater.
