Starting with a 2-fill, two 3-fills, and a 4-fill near collapse, there’s value by value action left in the bypass for only one value. Scroll back for the givens and trace out the bypass.
An x-fill lists the values to be filled in the line. The square bracketed effects are the clues and subsets placed in the line
Here’s the grid you take into box marking.
Box marking adds all candidates which are members of box slinks, strong links due to having exactly two of the same value in a box. Box slinks are marked in the same position at the tops of cells.
Some slinks are already marked. They are subsets, naked pairs, two values in two cells in a box or line. Column 3, c3, has a line slink. Its position is bottom right because the line is a column.
You can follow this trace on your same grid into box marking. The box marking trace is a list of boxes having box slinks for each value. There is no list for 5 or 8 because all value 5 and 8 clues are placed. The naked pair 69 in the NE box is the result of the value 9 slink combining with the earlier 6 slink, and the pair overwrites a 1 candidate, placing the other 1 in the NE box.
The follow up is fatal, but tracing is suspended because order of placements no longer matters.
Here is the grid at that point, when many naked pair subsets remain.
This week after x-fills, only the value 8 remained to complete the bypass. The week before that, the Kampelmann Hard 109 bypass reached a solution on x-fills alone. I was waiting for the next Washington Post 6-star when my Sudoku expert pal Gordon Fick suggested I look at the WP puzzle supplier AndrewsMcMeel’s website. Turns out, the site has a sample 6-star. What could be more typical of their 6-stars, for a last weekly post? So here it is. Do the x-fills first.
Systematic Sudoku 