When pieces are collected on a line, as they are on the b-file here, it is natural to wonder how you might take advantage of the arrangement. Or you might simply consider every check White can make and see Rb6+. Either way the point to see is that Rb6+ creates the kernel of a discovery, with White’s knight suddenly all that stands between his rook and Black’s queen. Usually—and here—a move that so creates the kernel of a discovery is really interesting only if it also helps create the other elements of a discovered attack by forcing targets into place. In this case Rb6 also forces the king to find a new square, and its choices are limited to a8 and c8. Whichever move Black makes, the king ends up on the same color square as White’s knight and is checked on the next move with Nc7+ or Na7+. After the king avoids the check, White takes Black’s queen.
Notice the style of thought: take note of moves that put one of your pieces onto a line with another that can unmask it; consider whether any of those moves also force a reply from the other side, and especially whether they force a target into place.