This is the inverse of an AND Gate.  You see in the table V1 is reversed from the AND Gate table.  V1 is moved up above both switches.  Below the switches is a ground. 

If both switches are on, the power goes to the bottom ground since V1 has some resistance.   So V1 is 0.   If both or either switch is off, the bottom ground is an open circuit and voltage goes through V1.   Below both power supplies are Low = 0, threshold = 2.5, High = 5.   So Both power supplies above 2.5 will read a 1 and V1 will get a 0.