A squeeze play that transfers the guard from one defender to the other.
You are aiming to make the four remaining tricks at no-trumps (or with diamonds as trumps) and have the lead in dummy. North has two menaces, the ♠J and the ♥Q, but each opponent guards one of the menaces.
If you simply cross to the ♥A and cash the ♦A, there is no squeeze because West keeps spades and East hearts. If, however, you lead the ♥Q (covered by the king and ace), the heart menace becomes the ♥10 and responsibility for guarding the suit ‘transfers’ to West.