So given the density operator for the state. He uses the no signaling condition which says that "If we have an entangled state between Alice and Bob and Alice measures the state in any basis pair the output for Bob remains the same i.e. he gets maximally mixed state in both cases and he isn't able to tell in which basis did Alice measure."
I am aware of the condition but ain't sure about its application on the above two qubit density operator. Can someone help me around this. So what i get from this is he just rotated the state in +z and -z axis and similarly in +x and -x axis and put the results in the no signaling equation and he got the relation between the parameters. Am i correct?