The Underarm Box is a variation
of the Box in which one of the vertical throws of each cycle is thrown
under-the-arm, meaning both vertical balls are on the same side of your
body. The position alternates sides every cycle, with a normal Box throw
connecting the two sides. The Underarm Box is similar to the Burst Box,
except that this trick lacks an under-arm catch of any vertical ball.
To learn the Underarm Box, start with one ball in each hand. Cross your non-dominant hand underneath your dominant hand and make a vertical throw. Simultaneously, make a horizontal pass from your dominant hand while rapidly uncrossing your non-dominant hand to position it on the non-dominant side of your body, where you will catch the pass. The vertical ball will then be caught by your dominant hand.
Practice this on both sides. To add in the third ball, start with two balls in your dominant hand and one ball in your non-dominant. Make a vertical throw from your dominant hand relatively close to the center of your body, and then, as that ball begins to descend back toward your dominant hand, cross your non-dominant hand underneath your dominant hand and make a second vertical throw at the very outside of your body. As in the previous step, you are also going to simultaneously make a horizontal pass from your dominant hand which will be caught by your non-dominant hand after it uncrosses. Both vertical balls will then be caught by your dominant hand.
Again, practice this on both sides. To run the full Underarm Box, just make another vertical throw from your non-dominant hand (at this point it is uncrossed) paired with a horizontal pass from your dominant hand. This will set you up for the next cycle, with your dominant hand catching the second vertical throw from the previous cycle and then crossing under your non-dominant hand to make another vertical throw.