Their angle doesn't change while going through the specimen. The rays that are diffracted are the two in the middle. They go into the specimen at one angle and come out at another angle. This is what diffraction means. The diffracted rays miss the phase plate and go through the little hole in the middle.
Uninteresting things show up dark because they go through the phase plate.
Interesting things show up bright because they don't. The original figure was in color and the two types of rays were color coded black and pink. Various things can cause this phase change. I'm sorry if that's all wrong. I am doing Ph.