Tier 3
The rule of crowding
Count in the other direction — when three rows can only be served by three colours, those colours belong to those rows and nowhere else.
Simple undercounting starts with colours and counts rows. Crowding starts with rows and counts colours. Same pigeonhole, opposite handle, and it fires in positions where the other version finds nothing.
Take a set of rows that still need queens. Collect every colour that appears among their surviving squares. If k rows draw on exactly k colours, those colours are spoken for: each of the k rows takes one, and none of them is left over for anywhere else. Every square of those colours outside the k rows can go.
Reading the board for it
You are looking for thin rows, not small regions. A row with two surviving squares in the same colour, next to another row with two surviving squares in that colour and one more — that is where crowding lives.
row 3 → colours {amber}
row 6 → colours {amber, teal}
row 9 → colours {teal}Three rows, two colours. That is not a crowding pattern — it is a contradiction, and it means you made a mistake earlier. Which is the other use of this technique: it is the fastest way to catch your own errors on a big board.
Why it is tier 3
Not because the logic is hard, but because you have to look for it. Nothing on the board points at it. You have to decide to enumerate row-colour sets, which is why solvers who never learn it still finish most boards — just slowly, and never the hardest ones.