I was reading this phys.org article and it sent me on a long wikipedia walk reading about surface plasmons and related materials.

Then, after some googling, I found this video. In the video, it is mentioned that reactions that take place on the ligands result in a shift in the dark spot on the detector.

The narrator says that the resonance at the surface is very sensitive to environmental changes, but I don't fully follow. How and why does binding to the ligand cause this shift in angle? How is it connected to to the plasmonic resonance at the material's surface?

EDIT: I realize that my question is more about the application of SPR in biochemistry than actually what the phenomena is.