How can AND check for a 0?

The circuit needs to check two things:

  • A must be 1.
  • B must be 0.

AND can check whether two signals are 1. It cannot check for a 0 directly.

So B goes through NOT first.

When B is 0, NOT changes it to 1. AND now receives two 1s:

  • one from A
  • one meaning “B was 0”

Both conditions are true, so the circuit outputs 1.