Slam your back foot down on the tail of your skateboard as hard as you can. At that moment, you want to also jump into the air, off of your back foot. This part is key, and takes practice. The trick is in getting your timing right. You want to slap the skateboard’s tail down, and as it hits the ground, jump off of that foot into the air. Make sure to pull that back foot high into the air. It's a quick, snapping motion.