Squatting down as if sitting into a chair with a wall nearby to catch your balance, you drop into the bottom position and explode straight up into a jump. The wall is there purely as a spot for stability, not something to push off, so you can focus on the plyometric drive out of the squat.
Log your plyo sit squat (wall) sets in 1RM.fit and watch your estimated 1RM update automatically as you get stronger.