This video clip is a colored drawing of a black hole and the disk of material that surrounds it. We see the disk rotate around the black hole. A bright yellow blob appears on the disk, and we follow it as it rotates around the black hole. We also see the spectrum of emission from this blob, measured on an energy scale in KeV. The intensity of the line (represented by the height of the line on the graph) changes as the blob moves around the black hole. The energy of the line also changes.
The energy and intensity of the emission line change as the blob moves around the black hole because of the Doppler shift caused by the blob's motion, and because of the the shift due to the strong gravity of the black hole.
After a time we see a second blob of material further out, and we can follow its spectrum. Comparing it to the first blob, we see that it does not range in energy as much as the first blob.