Explanation

This is a very common combination. You can think of the blocking piece being pinned to the forking square.

Examples

Black can open the line for the rook from c2 to g2 by exchanging queens with 1...Qxd3. White has to recapture with the knight 2.Nxd3, which was blocking the rook. Black can then win with 2...Rxg2+, forking king and bishop.

Black can open the line for the e-pawn with 1...Bxd5+. White has to recapture with the pawn, which was blocking e4: 2.exd5, enabling the pawn fork 2...e4.

Sometimes opening the line can be a temporary sacrifice:

Black can sacrifice a knight with 1...Nf3+! White has to capture the knight with 2.Rxf3, opening the line for Black's queen from e6 to e1. Black can then win a rook with the fork 2...Qe1+. Black has sacrificed a knight, but won a rook, so won the exchange in the end.

Related motifs

A line can also be opened for the fork attack (instead of for the fork move).
It is also possible to open a line for checkmate, or for a capture (use the pin).