[saurik]

It isn't the same as "masking out a bit" when you are subtracting 64 from 63 to get 127 (the behavior of C-?). I think it is equivalent to Xor-ing it, but I didn't want to go into that for various reasons, including it just being overly complicated (and, in fact, edited that statement three times in an attempt to find something that was all of true, expository, and easy to understand; but, I apparently didn't optimize for "something everyone would appreciate why it was said the way it was said if they liked adding information but hadn't paid as much as attention to all of the examples as I had been forced to while drafting the comment").