Grid technique · Advanced
Block Counting
The pigeonhole check behind every locked-set deduction: if m items are confined to fewer than m possibilities, the puzzle has contradicted itself, and if they are confined to exactly m, that is a locked set.
Why it works
Every category holds exactly the same number of items as subjects. Squeezing more items than possibilities into one space is never valid, which is what makes a tight count worth checking.
Trigger
A cluster of rows or columns whose candidate counts look unusually tight relative to how many of them there are.
Common mistake
Not noticing that a locked-set search doubles as a sanity check — a count that comes out too tight usually means an earlier mark was wrong, not that a rare pattern was found.
Example
If four items are between them confined to only three possibilities, something upstream has gone wrong. That combination cannot exist in a valid solve.