IDK. I watch Andreas' video on BS 270's when I was trying to learn and it made it worse for me. IMO there is a huge difference between 270 off of a box and 270 off of a rail. When I tried to scissor on a rail, I couldn't get beyond 90 off, probably because I wasn't committing with my upper body. You can scissor to stop your rotation, so if you scissor without rotating into the spin, you won't spin.
My friend just told me to forget about trying to scissor and just turn my head uphill once I was on and look at the start of the rail. 2nd attempt, it worked after hundreds of failed attempts. I think the scissor only works when it helps you get you shoulders rotating into the spin. I have seen guys spin off of boxes without scissoring too, but I still have trouble now spinning off of a box.