Sandboard have a tutorial how-to-use, that is shown after-purchased. It can be replayed as follows:
Menu > Settings > Replay Tutorials > Sandboard
In-game description Edit
"Crafted long ago in the canyons, Izel has equipped it with special bindings. Use this durable board to wallride! " ― Izel's Workshop
If the sandboard is purchased, the player can wallride. This game mechanic allow players to flip their boards and vertically ride canyons straight up, over chasms, and at angles. It's a bit trickier to master than straight backflips, but the combo possibilities make these new tricks completely worth it.