[wruza]

Because to negate you invert (so & = 0) and add one, which overflows former zeroes until it meets a former one, which flips, so & gives 1 there. Former zeroes are how even a number is.

  01100 (12, 2 bit even)
  10011 (inv 12, & = 0)
  10100 (inv 12 +1 aka -12)
  00100 &, 2 bit even