From the video:
“The reason is that they don’t want to have oxygen getting in during the early startup because an oxygen rich environment (or oxygen rich pockets) could damage the engine. Hydrogen is a lot less chemically active, and less likely to corrode critical parts of your engine at an inopportune moment. So that hydrogen exhausts out the back of the rocket and, then being hydrogen, it rises up very quickly. And at some point, one of the sparks from the will set it on fire and you will have this ginormous[sic] fireball which runs up the side of the rocket.”
I didn’t catch the name of the spark generators. Something like “rofies”. No doubt a nickname for something more technical. I couldn’t Google up any likely names. Perhaps it’s just slang for “pyrotechnics”.
Note that hydrogen burns with a pale blue flame, so we’re apparently seeing other stuff being burned by the hydrogen fire. The surface of the rocket certainly gets scorched, and it may be that material that produces the orange flame. It may be a number of other things as well; whatever happens to be down there.