I was recently working on something similar to this. I was unable to find much detail in the documentation about what the control matrix format is, but with a little experimentation I got the results I was looking for.

For cubic NURBS, the matrix is a 4×4 array of control points with 3 planes for the X,Y and Z coordinates.

To do the "dent" effect you want, you can make the base control matrix a plane at Z=0, then push the Z in the negative direction for the middle 4 points (indicated with X).

Something like this:

(-1, 1, 0) (-0.333, 1, 0) (0.333, 1, 0) (1, 1, 0)

(-1, 0.333, 0) (-0.333, 0.333, 0) (0.333, 0.333, 0) (1, 0.333, 0)

(-1, -0.333, 0) (-0.333, -0.333, 0) (0.333, -0.333, 0) (1, -0.333, 0)

(-1, -1, 0) (-0.333, -1, 0) (0.333, -1, 0) (1, -1, 0)

I stored the control matrix in a [jit.matrix 3 float32 4 4] then used four setcell messages, like [setcell 1 1 plane 2 val $f1] to modify the Z coordinates of the middle 4 points.

Hope this helps!