How does this bitwise operation check for a power of 2?

Any power of 2 minus 1 is all ones: (2 ^N – 1 = 111….b)

2 = 2^1.  2-1 = 1 (1b)
4 = 2^2.  4-1 = 3 (11b)
8 = 2^3.  8-1 = 7 (111b)

Take 8 for example. 1000 & 0111 = 0000

So that expression tests if a number is NOT a power of 2.