What offensive possibility do you see for White? No doubt the alignment of the Black queen and knight on the long diagonal is self-evident by this point; likewise White’s ability to impose a pin with Bd4. Black’s knight seems then to be attacked (by White’s bishop and queen) more times than it is defended (by Black’s queen); so it appears at first that the pin is a winner. But before imposing any pin—and especially a mere relative pin—ask what your opponent could do in reply. You are looking in particular for time-consuming offensive threats he would be able to make with either his pinned piece or his screened piece, and especially any checks either piece could give. Here Black’s knight could exit the pin with check: Nf3+. Of course White just takes it with his bishop or queen, but this costs White a move (the priority of check); and now Black takes White’s bishop, which has been left loose, with his queen. The pin therefore doesn’t work. It is another example of a case where Black moves the pinned piece with gain of tempo, which allows the screened piece to go on the offensive next move.