The channel is of course not specific to Synfire, but there's a ton of information which will be of interest to many Synfire users. They don't just teach music theory in isolation, but instead show actual uses in context.
Warning: They assume some prior knowledge of theory on the part of the listener, so if you don't have that then much of the material will be over your head. But it's not super-esoteric stuff either. It's all on a practical level, with the goal of helping people write better songs.
I plan to enter some of the progressions they use into Synfire (IOW to save them as Harmony parameters) and use them as the basis for compositions/songs/productions.