Where does White have the kernel of a discovered attack? Not on any file, of course, but along the sixth rank. It isn't easy to see because at present the rook has no target once unmasked; but White would have a discovered check if Black’s king could be drawn onto h6. Experiment with any checks White can give using other pieces—or pawns. Here h5-h6+ checks the king; it also puts a pawn on the square where we want Black’s king to go, inviting a capture there.
Black has two options: take the pawn or move the king to the back rank. If he goes the latter route, consider first what checks would be possible and you find Re8—which is checkmate. (See how the pawn on h6 and the bishop on g6 together seal off all the king's flight squares on the seventh rank.) So instead Black will have to play 1. ...Kxh6. But now he has put his king on the same line with White’s bishop and rook and has made it prey to a discovered check. White merely needs to find something good for his bishop to spend two moves doing, and he finds it in Be8+. After Black moves his king out of check, White plays BxR, winning the rook.
Realistically, the most likely way you would see the tactical idea here is to start by examining the h5-h6 check as a matter of course; then you see that after Kxh6 you have a discovered check arranged—and that if the king instead retreats you have mate.