South has a tough decision here.

Leading a Heart away from ♥ Q x x is clearly a mistake as it may give a trick away

Given you've just decided not to lead a Heart because you probably have a natural trump trick if you don't then there is no point in leading your Doubleton Spade hoping for a ruff because you can't have your cake (natural Trump trick) and eat it (Ruff a Spade)

So you're going to lead a Minor Suit and you aren't going to lead a small card from underneath an Ace (it's too dangerous against a Suit contract)

OK - we know you are going to lead the ♦ A or the ♣ A. Is there anything to choose between them?

We know partner is unlikely to be short in Diamonds (we have four). Although partner may be short in Clubs they have at most one Heart so are unlikely to get a Ruff

In a photo-finish the ♦ A looks the better bet and so it turns out

If you lead the ♣ A Declarer can discard a losing Diamond on the ♣ Q and makes their contract