How do we turn off the rightmost bit off in a number?

We "and" the number with one less the number.

Lets suppose the number is x then to turn the rightmost bit off in the number we do a x&(x-1)

Example

14 - 00001101

And it with 13 - 00001100

14 & 13 would give 12
00001101
& 00001100
---------------------
00001100

Bitwise representation of 12 is 00001100.