The True Box (unofficial name),
also called Box with a Lid, is an extreme shape distortion of the regular
Box pattern in which one ball is caught and passed in the air to create a
rectangle (with the ball being manipulated at the corners of the shape).
Despite its bizarre appearence, the True Box actually has the same siteswap
as the normal Box, with some of the horizontal passes performed vertically
To learn the True Box, start with one ball in each hand. Simultaneously throw both balls vertically, then quickly reach your dominant hand over and grab the ball thrown from your non-dominant hand. If done correctly, this ball should be caught before the other ball reaches its peak. As that other ball descends, move your dominant back down and catch it.
This aerial catch will likely feel very awkward at first, but with practice you will eventually master the timing. To add in the next throw, you are going to again start with one ball in each hand, and then perform the same vertical throws and catches you did in previous step. This time however, you are also going to throw the ball you caught in the air over to the other side of your body. This will clear space for your dominant hand to catch the other vertical ball. As the ball that was caught and then thrown by your dominant hand reaches the other side of your body, bring your non-dominant hand up and catch it.
Once again, practice this until you feel comfortable with the throws. To add in the third ball, start with two balls in your non-dominant hand and one ball in your dominant. Make the same throws and catches as in the previous step, but this time make a vertical throw from your non-dominant hand before using it to catch the ball in the air (this happens after your dominant hand catches and then throws that same ball). As a general rule, all vertical throws made in this pattern should be the same height. As that last vertical throw reaches its peak and begins to descend (your non-dominant hand should have just caught the ball in the air), you are going to drop the ball from your non-dominant hand and then bring it back down to catch the vertical throw.
The step described above contains all the throws and catches performed in the air, which together make up the primary pattern. However, to connect the various cycles, you will also need perform a normal horizontal pass/vertical throw combination. To add this in, you are going to repeat the process from the previous step, and then, as the ball that has been in the air the entire time begins to finally descend toward your dominant hand, you will make a vertical throw from your dominant hand. This will clear space for that hand to catch the other ball. As the ball just thrown begins to descend, you are going to make a horizontal pass from your dominant hand and a vertical throw from your non-dominant hand (this throw should be made close to the center of your body).
The animation above demonstrates one full cycle of the True Box. To run the full pattern, just make another pair of simultaneous vertical throws as that last throw from the previous cycle approaches your non-dominant hand. The True Box is not an easy pattern, and will take time to learn, especially if your goal is to create a clean rectanglular shape.