A loose piece can make a fine target even when it lies in its original position on the back rank. Here Black’s bishop on c8 is loose; White can attack it with Qc1. But this only is interesting if the attack can be paired with a simultaneous threat elsewhere. The key point to see is that from c1 White’s queen would be aimed at the Black knight on h6. That knight already is attacked once and defended once, which is to say that it’s as good as loose. Adding an attack against it by the queen amounts to a queen fork of Black’s bishop and knight. Black probably responds with the interposition Nc6, allowing White to play BxN next move.

We have seen many times the importance of being aware of loose pieces. But it likewise is important to be aware of pieces attacked once and protected once; they often make targets for forks that are just as good as pieces with no protection at all.