These are basically the sum of the absolutes values of the partial derivatives of the voronoi function (more accurately the finite difference approximation of them).
2D texture version:
Better suited for 3D:
There is some similarity to the rotational version, but that would produce non-uniform crack thickness (zero at the origin of the texture coordinates and infinitely increasing by the distance of the origin).
You could also achieve these by using only one additional voronoi, but that would result lower quality (actually, the more additional voronoi vectors you use, the better will be the quality, but that depend on how well the coordinates are shifted for each of them).
The laplacian kernel method also shares some similarity, but (du to more calculation of the voronoi funcion) has higher quality (at the cost of performance).
This is simpler and faster, and quality is acceptable if not zoomed in too much. If you want to get uniform crack strength, add a grater than node before the output of the node group.