2 Strong Links
2 Strong Links is the general case of the strong-links family — two conjugate pairs on the same digit in any combination of houses where one cell from each pair sees the other. The bridge cells can't both hold the digit, so at least one opposite endpoint must, eliminating the digit from any cell that sees both endpoints.
What are 2 Strong Links?
2 Strong Links is a single-digit pattern built from two conjugate pairs — two houses where a particular candidate appears in exactly two cells. The houses can be any mix of rows, columns, and blocks, provided one cell from each pair shares a line with a cell from the other. Those two shared-line cells are the bridge: because they see each other, they can't both hold the digit. Since each conjugate pair must place the digit in one of its two cells, and the bridge can't supply both, at least one of the other two endpoints has to contain the digit. Any cell outside the pattern that shares a house with both of those endpoints can't also hold the digit, so it gets eliminated.
How to spot it
Like the other strong-links patterns, 2 Strong Links doesn't jump out the way singles or locked candidates do — you find it by scanning one digit at a time. Pick a candidate digit and look for houses where it's restricted to exactly two cells. When you find two such houses, check whether one cell from each pair shares a row, column, or block. If they do, you've got a 2 Strong Links pattern: the shared-line cells are the bridge, and the other two are the endpoints. The elimination lands on any cell that sees both endpoints. The specific sub-cases have their own names — when both pairs sit in parallel rows or columns it's a Skyscraper, and when one is in a row and the other in a column it's a 2-String Kite — but the remaining combinations, where at least one pair lives in a block, are all called 2 Strong Links. They turn up regularly in harder puzzles once the easier techniques have run their course.
The logic
Each conjugate pair is a house where the digit has exactly two candidate cells, so one of them must be the digit. In a 2 Strong Links pattern, one candidate from each pair sits on a shared line — the bridge. Because the two bridge cells see each other, at most one of them can be the digit. If the first bridge cell is the digit, the second pair's digit falls on its opposite endpoint. If the second bridge cell is the digit, the first pair's digit falls on its opposite endpoint. And if neither bridge cell is the digit, both opposite endpoints must be. In every case, at least one opposite endpoint contains the digit, so any cell that sees both endpoints can't also hold it. The same reasoning holds regardless of which combination of houses the two pairs live in.