Suppose start from X=1, Y= 1
A and B is also 1 respectively
now X will be 0 as nor(1,1) will be 0
this 0 will also be fed with B and now when B is 1 nor(0,1) will be 0 i.e. Y = 0
this A is 1 Y is 0, nor(1,0) is 0 i.e X is 0
now X is 0 and B is like 10101010101010 in this sequence
this time B has to take 0
Nor(0,0) will be 1 i..e Y will be 1
X will be 0 and Y will be 0 and so on