I am confused about one point I have read the following paragraph from the networking book.
“the twists in the twisted pair cable are used to avoid the unwanted signals. For example one twist, one wire is closer to the noise source and the other is farther; in the next twist the reverse is true. Twisting makes its probable that both wires are equally affected by the unwanted signal. This means that the receiver which calculate the difference between the two receives no unwanted signal.”

Now ok I understood the purpose of twists but I am confused about how receiver will calculate the difference when it will receive the signal?. How unwanted signal will be eliminated ? 
Another thing that I want to make clear is ,  I am beginner please provide such an answer that can be understood.


Question: Word for the opposite of "hypochondriasis"
Answer: You could perhaps make up a new word based on the term prosopagnosia, which refers to the neurogenic inability to recognize people's faces. The term would be pathagnosia, the inability to recognize sickness. However, this inability would in principle apply equally to others' illnesses, so that a sufferer of pathagnosia would be just as bad a doctor as a patient.