Assign gradient to t[1] and t[-1], considering the direction of AA[2] and AA[-2] respectively.
The blue tetrahedrons are assigned gradient (bold edge) as shown on the left. Note that "initial positions" of t[2] and t[-2] are also determined (white tetrahedrons), which are moved to AA[2] and AA[-2] respectively later.
The 2nd derivative is computed by the following rule: change value if gradient changes.
As gradient, t[1] could assume either x2x3x4 (left) or x1x2x4 (right). And t[2] is closer to AA[2] when t[1] assumes x2x3x4.