This begins a four post review of Puzzleproof’s Extreme Sudoku 2. The pre-selected puzzles are every 23rd puzzle starting with 7. All of the puzzles are solved in the bypass or in box marking. The review provides full tracing, and highlights some Sysudoku Basic moves. This post covers review puzzles 7, 28 and 49.
On Pzpfx2 7, suppose this is your reading grid as the trace tells you that W7 => (E7, E5), and you add 7r5c7. E7 has no effects, and its up to you figure out where E5 goes and why. As a trace reader, you focus on the W7 placement. What changed? The small square. You review what you do when a small square completes. Oh yeah. The r5 5-slink, and the c7 5-slink cross hatch E to net E5 at r5c9.
Now look ahead. E5 resolves the r5 4-fill with W1, which works with the small square for W8, and it’s soon over. Here’s the trace:
Next we look at Puzzleproof Extreme 2 #28. After a bypass flogging, it begins to crumble in box marking on the 7: list. The fatal blow is a 3-fill, whose first cause fills a wall, leaving two clues in reserve.
This shows the 
effects of this first cause to be four clues, with the collapse continuing from SW8t above.
Puzzleproof Extreme 2 # 49 is a replay of #28, the collapse triggered by the first effects of a 3-fill in the 9: list of box marking.
Here’s the grid after the bypass. Next post, it’s #’s 70 and 91.
Review alert: Soon after Puzzleproof’s Extreme Sudoku 2, I’m expecting to review Stephan Heine’s Sudoku ultrahardcore 1, which, except for the title, is in German. It’s at the suggestion of commenter Alex, who provides links to booksellers for this book at the end of the Reviews page.
Systematic Sudoku 