[pieceofpeace]

You are right that "the purpose of many the constraint techniques seem to be aimed at creating puzzles that are amenable to humans, not solving them"

I shared the link because I found his sudoku generator and the constraint methods interesting.

> That would be backtracking.

Let me quote the first para in full:

  The solver uses depth first and/or breadth first tree search
  with constraint propagation to prune the search for the next
  best move (forms of forward checking.) There are space/time
  tradeoffs between depth/breadth first search and the constraints
  used; sudoku(1) has options to control the combinations.
  The common characteristic for all constraints, here and elsewhere,
  is that they avoid trial and error. Its fine for a computer
  to guess and backtrack but a definite breach of puzzle manners
  to require a human to do so.

I could be wrong but, yes, it does use backtracking to find the constraint that can give a number for an empty cell but it never has to change a number it has put in a cell. That differs from the trial and error approach that moves forward by guessing values and checking if it leads to a valid solution.

[crux_]

The constraint-finding is for generating puzzles that are easy, hence the comment about puzzle manners.