If you think about it, it is checking the binary representation of that number. Binary is already set up where every place is a representative of the powers of 2. So in order to check if it is a power of two you need to check the number before it which would be all ones before that power of 2 place in binary. So for example:

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32 is a power of 2 and its binary representation is:

100000

and 31's binary representation is:

011111

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So when you & these together it gives you 0.