A heavy bypass leaves a swordfish on an imbalanced grid. One ALS_XZ or a Single Alternate Sue de Coq bring a collapse.
Almost a collapse in the bypass.
The marked grid is dominated by values 2, 3, 6 and 8. The swordfish rows are 2,3 and 7, with crossing lines 2, 3, and 6. All three of the swordfish lines are line slinks, making it visible in the pencil marks as row 7 is marked.
When the 5 removals are made, a dead swordfish remains on rows 1,3 and 8.
One route to a quick collapse turns out to an ALS_53.
A second route to a quick rout is a Single Alternate Sue de Coq, which normally takes a trial of one alternate being missing.
Both the c7 bv 35 and the SW ALS 358 insist the SWc7 can’t have 3 and 5. But 3 and 5 can’t be missing from SWc7. That would make 3r8c2 and r3c3 both solve to 3, forcing two 5’s into c1.
So SWc7 = 7(3+5)(2+6), and r8c2 must supply 2 or 6 to the SW box, removing 3r8c2.
That’s all it takes. The collapse is fast.
Wex 439 solution:
Systematic Sudoku 