This trick is the same as The Box except that the horizontal throws are replaced by Slam throws.
Start with two balls, both in the same hand. Throw one straight up to about eye level, after it peaks & starts to fall, raise the other hand up past it & Slam the other ball over it & into the other hand, in the same movement Claw catch the first ball. When you throw the first ball up, make the throw from a position close to the centre line of your body. When using all three balls try to keep the two columns within hip width apart. If the two columns are too far apart you will have to stretch your arms out to make the slam throws. The further your arms are stretched the less control you will have over the throws.
Practise this on both sides.
After that move on to three balls. If you can already do a standard Box start from that & practise adding in Slam throws, make sure you don't break from the box pattern, it is easy to go into two in one hand rather than passing the slammed ball straight back. As you slam also make sure you throw over the vertical balls, not in front of or behind. Work up to making every horizontal throw a Slam.
If you can't, start with two balls in one hand & one in the other. Begin with a vertical throw from the hand holding two. Just as you did in the two ball exercise, slam the second ball from the same hand over the top of the first. At the same time that you throw the slam, throw the ball in the opposite hand straight up to eye level to empty that hand in time to catch the slam. Catch the other ball, you should finish with two balls in the opposite hand with which you started.
Now do the same on the other side.
Work your way up to a continuous pattern. Start with one slam, then two, then three & so on.
As you do more & more throws you will probably find yourself losing control of the two balls that are thrown vertically. Concentrate entirely on these two balls. Make sure they travel straight up & straight down with no sideways or back & forth movement, they should be caught from the exact position that they were thrown from. Remember to keep the two columns close together so that you don't have to stretch out to slam a ball over another. Also after you throw a slam make sure you bring your arm all the way back down to waist height. The further you stretch, the less control you will have.