1Definition: TDP ("Technique des pistes" in French, roughly translating to "Tracks Technique") is a network technique, developed by Robert Mauri�s in 2011, "using only the basic techniques so that it is easy to use for sudokists with little experience in advanced techniques." Sue utilizes a subset of TDP: "anti-tracks." We choose a BVC, assume that one of its candidates is true, coloring it blue. Then, using medium-level techniques, we see what else would be true in other cells across the board if that first digit were true, and color those candidates blue as well. Then (depending on the results of the above) we may do the same with the other digit of the original BVC: coloring it, and the candidates it renders true, orange.
A TDP with a contradiction and a circle. Starting in C1, assuming 4 to be true raises a contradiction in F9 (no digit left to place), so blue digits resolve, and A6 is a 9.
2Look For: A starting BVC (or CP) especially if it appears that solving it might have a large domino effect.
3Consequence: There are five possible consequences:
The assumption solves all the cells. Convert all candidates of this color to Big Numbers: the puzzle is solved!
The assumption reveals a contradiction. Convert candidates of the other color to Big Numbers.
Both assumptions resolve to the same candidate in a cell. Convert that candidate to a Big Number in that cell.
An uncolored candidate sees its matching digit in two different-colored cells. Erase that candidate from that cell.
An uncolored candidate sees its matching digit in one color and has one opposite colored candidate in its own cell. Erase that candidate from that cell.
4Why it Works: It works because the initial BVC must resolve to one of two values. TDP coloring allows us to look at the consequences of assuming each one to be true in turn.
5If, after the inital assumption and coloring, all unsolved cells have a blue candidate, then our initial assumption was correct, and the blue candidates solve the puzzle: convert them all to Big Numbers!
6Otherwise, we take the second candidate of our initial BVC cell, assume it is true, and color it orange. We go through the same process of determining what else would be true if our orange candidate were true, coloring them orange as well. If all unsolved cells have an orange candidate at this point, then our second assumption was correct, and the orange candidates solve the puzzle.
7If the puzzle remains unsolved at this point, we need to see if either assumption resulted in a contradiction. For example, if the blue candidates resulted in no candidates remaining for a cell, then we know the blue assumption was false, and we can resolve all orange candidates to Big Numbers (and vice-versa).
8If both assumptions resulted in coloring the same candidate in a cell (i.e. the candidate was colored both blue and orange) then that candidate is true for that cell (Sue will circle these candidates).
9Any uncolored candidate that sees its matching digit in both blue and orange can be eliminated.
10Any uncolored candidate that sees its match in one color and has one opposite colored candidate in its own cell can be eliminated. (Because in the first scenario the digit solves outside the cell, while in the second scenario the cell solves to a different candidate. Either way we know this candidate will not solve to this cell.)
11Let's go through our example, taking the first BVC on the board for our assumptions. We start by assuming candidate 1 is true for the cell, coloring it, and everything it makes true, blue. We use nothing more advanced than the Medium level techniques, assuming the blue candidates are big numbers as we go:
The 1 in C1 eliminates the 1 in C8, leaving the cell to be a 7.
The 7 in C8 eliminates the 7 in A8, forcing the A8 to be a 1.
The 1 in A8 eliminates the 1 in A4, leaving the 1 in B4 as the only 1 in block 4.
The 1 in B4 eliminates the hard-candidate 5 in B4, leaving the 5 in B5 as the only 5 in column B.
The 1 in A8 also eliminates the 7 in A8, making the 7 in A2 the only 7 in column A.
The 7 in A2 eliminates the 9 in A2, making the 9 in A6 the only 9 in column A.
The 7 in A2 also eliminates the hard-candidate 7 in G2, leaving the 7 in G3 the only 7 in block 3.
12We didn't find any contradictions in our first assumption, so we move on to our next assumption: that the 4 is true in C1, coloring it, and everything it makes true, orange. This time we won't go through the details of why each orange candidate is true assuming C1 is 4. You can verify them for yourself if you like. Two things to notice here:
Both assumptions (blue and orange) result in A6 being a 9. That is why it is circled, and why we can enter a Big Number 9 in that cell.
The orange assumption leaves no possible candidate for cell F9. The candidates shown in this cell already appear as orange candidates elsewhere in its house:
The 7 in F4.
The 8 in B9.
The 9 in H9, et al.
13Since the orange assumption results in a contradiction, we know the assumption is false. Therefore the orange candidate (4) in C1 is false [note that this does not make all orange candidates false], leaving the 1 to be true in C1, and then it follows that all of the blue candidates must also be true, and converted to Big Numbers!.
Prev: 
