Correspondent pairs, triples, and quads

Cell correspondence

First, an illustration with a "correspondent pair":

Elimination 1

Consider the following sudoku

Whatever Row 4 Column 1 is, it will eliminate that candidate from all of Box 1 except for Row 2 Column 3.

Therefore, R2C3 is the same as R4C1, so you can eliminate 3, 4, 8 from R2C3

Elimination 2

Box 1 forms a pair with R4C1. (Hence the name of the strategy)

Observe: R4C1 = R2C3 =/= Rest of box 1, i.e. It is impossible for 1 and 2 to both fit into R2C3, so the rest of box 1 must have either 1 or 2, which pairs with R4C1

So 1 and 2 can be eliminated from R5C1, R6C1, R7C1, R8C1, and R9C1 !!!!!!!!!!

Extending to correspondent triples

Ngl this is basically the Almost Locked Sets strat.