The box pattern can be seen as a synchronous shower (a fountain), which direction is changed at every throw.
In a traditional box pattern, the vamp crosses underneath the vertical paths of the other two "side balls".
In the inverse box, the siteswap stays the same and two balls (4s) are still thrown in straight lines, but the ball thrown from the right hand is going up and down on the left side and vice versa.
The ball is then thrown from above the 4 diagonally downward to the opposite hand.
The resulting downward throw is often known as a shuffle, giving the trick its name.