The submission by David Wagner, Philipe Biondi at https://bingweb.binghamton.edu/~scraver/underhanded/_page_id...

The state of RC4 consists of a random permutation of bytes. Whenever it outputs a value, it further permutes the state by swapping some bytes of the state. Th xor swap trick sets one of these values to zero, whenever RC4 attempts to swap the same item within the permutation. This gradually zeros out the state, until RC4 outputs the plaintext.