Kinda glad this got brought up. The backside spin is obv. your back down hill initially on the spin. as far as boardslides go however, when I was riding my hardest, sponsors and whatnot, a backside boardslide was if your back faces downhill and frontside is the easy looking downhill style. I get that skateboarding changed this at some point but even in transworld, etc. all the mags back in 2000 era, anytime your back is facing downhill its a backside boardslide. I will still call it that myself, altho when I coach these tricks ill call it by its skateboard name. I cant stand how you couldnt tell what kind of slide it is unless you see how the guy gets on the rail. can you take a still pic and name it anymore...? no, you can take a still of a "what I call backside boardslide" and it can be front or back depending, makes no sense.
I could care less what side you approach or if you jump straight on the rail, its backside if you initiate a backside spin, its frontside if you initiate a frontside spin onto the rail. How can you spin backside onto a rail and then its all of a sudden a frontside grind. Whatever, cant stand it but the skateboarder/jibber snowboarder changed terms and I have to submit to them.