The logic of the discovered check sometimes can arise in positions where the unmasked piece doesn't give check but makes a different threat—perhaps a threat of mate, or a threat against the queen—that nevertheless is so powerful that it must be addressed; meanwhile the mobile, unmasking piece remains free to make two attacking moves.
In this first position, White’s bishop and rook are in the natural position for a discovery. The rook is aimed at Black’s queen, a threat so powerful that unmasking the rook works much like a discovered check: White will get to make two unfettered moves with his bishop while Black saves his queen. White doesn't quite have the same flexibility in attacking the queen that he would have with a discovered check, because unlike a king the queen sometimes can move out of harm’s way and guard against whatever threat the mobile piece has made. But that doesn't spoil White’s possibilities here. He looks for a two-move attack he can launch with his light-squared bishop, and sees that Black’s rook is loose and lies on a light-colored square. Ba6 thus attacks the rook on move one and wins it on move two (unless Black plays QxRe1, preferring to lose a queen for a rook rather than losing a rook outright).