The incrementer is based on a fast two's complement algorithm (which computes the negative of a number). If you delete the FILT(s) in NOT mode what you're left with is the two's complement algorithm.

Because a two's complement algorithm is functionally equvalent to "flipping the input bits + 1" you can easily turn it into an incrementor or decrementor: if you invert the intput you undo the implicit bit flip so what you're left with is the "input + 1" part and you have an incrementor, and if you invert the output you effectively compute "-(-input + 1)" which is equivalent to "input - 1" so you end up with a decrementor.