explain backside boardslide body positioning once you're on the feature, on a skateboard and a flat rail it's not hard but god damn I hate being trash once strapped in
Are you slipping out on the rail?
If you can do boardslide on a skate you can for sure do it on a snowboard. You probably have a mental block about doing it just because you're strapped in and can't jump off if your body weight is off center.
Keep your arms parallel with the rail the whole time, with your front arm pointing towards the end of the rail. This is key because your arms and shoulders are what keeps your weight balanced so you don't slip out.
When you air on, rotate your lower body, almost like a shifty, so you lock into boardslide, so turn your legs 90°. Keep it nice and low and tucked, no straight legs, you'll bail. Keep you head low and pointed forwards looking to where you front hand is pointing
Once on the rail, keep your weight even over your bindings and your knees bent. It's kind of hard to explain, but try and keep your weight "flat" on the board, as in the board flat on the rail, without shifting your weight to either toe or heel edge.
I find frontside lipslides easier than backside boards. It's the same trick positioning, except you have more time to feel out your balance and find the balance point to lock in on the rail when you air on to it as you rotate your legs into boardslide position.