If West discarded from a black suit when South cashed the final heart, the position would progress to another squeeze, of the simple squeeze variety. The reason that this does not occur after the Q is discarded on the 9 is that a squeezed position requires that at least one threat sit "over" the squeezed opponent. After West discards, say, the 10 on the 9, dummy discards a small club, keeping dummy's 9 as a threat sitting over West's Q. Now South can cash the A, the K and the 9 to effect another squeeze, a simple two-suit squeeze in spades and diamonds, against West.