[jonsen]

  1110 + 1111 = 1101
  1101 - 1110 = 1111
  1101 - 1111 = 1110

Overflow doesn't matter if you just want to swap.

[Sacho]

(Assuming two's complement arithmetic, as amelius pointed out - I don't know if all programming languages and platforms use it)

[mcintyre1994]

That's a really smart observation I hadn't made, thanks for explaining!