30-line mechanical television

[Rinehart]

This is about the first play ever produced on British television, The Man With the Flower in His Mouth, in July of 1930 using the Baird 30-line mechanical system. There was only one camera used, and since it was bolted to the floor, in order to go from one character to another, one actor would have to move off camera and let another one take his place. During this transition, one of the stage hands would place a board with a checkerboard pattern in front of the camera, because otherwise during the changeover the equipment would lose synch and the image disintegrate into a bunch of squiggles. My question is, why would it do that? And how would a checkerboard prevent it? (During the pre-production phase they had used a board of uniform colour, but found that it didn't work.)