Next, assign gradient to tetrahedron ...
Then, "initial position" ...
The "blue" tetrahedrons are assigned gradient as shown on the left. And their values are shown on the right. For example, t[1] is assigned gradient x2x3x4 and the 2nd derivative becomes "U" since the gradient value changes.
The "white" tetrahedrons at ends shown in the left figure are the adjacent tetrahedrons t[2] and t[-2]. They are moved to the position of AA[2] and AA[-2] respectively later.