So to perform this glitch, you place/bring a shulker on the ground, you then place a piston facing the shulker 1 block up and to the left of it.  Then place a lever on the piston. Place a wall 3+ blocks tall in front of the shulker. Then flick the lever, put a gravity-affected block on the piston arm, and flick the lever again. The gravity-affected block will go into the piston arm, wait a bit for the shulker to lower and it will go into the ground.