Vestigial modulation is a variation of AM single-sideband modulation where the lower frequencies are also transmitted on the other sideband. It's related to the fact that the video signal has a lot of low frequency content that is not readily transmitted by full single-sideband modulation............................
I never really understood vestigial modulation, I was looking at the squirrels in the college playing field when that was the subject.
I remember watching some of the wildlife on my college playing field but it never involved squirrels.