When a gravitational wave reaches the interferometer on earth, it will shrink one arm and stretch the other arm. The distance between the mirror and beam splitter will change in each arm. Therefore, the two laser beams will not be completely out of phase when they recombine. Instead, the peaks and valleys in the two waves will overlap. And as a result, when the beams recombine, some light will leak through the beam splitter and be recorded on the detector.