AJ10
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A
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x
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J9x
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You need two more tricks to make your contract. You can lead a spade, and if LHO plays small, it is perfectly safe to finesse spades, possibly setting up a spade trick for the overtrick.

The problem is if LHO plays a spade honor.

Principle #1. Opponents are more likely to play second-hand-high in the endgame. You have, after all, squeezed out their small cards.

You win on the board, leaving this postion.

J10
--
A
--


--
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J9x
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What now? It depends. If there are two clubs out, you will probably cash your ace of diamonds. This reduces your chances of an overtrick, but it guarantees the contract.

Principle #2. Declarer will probably play safe for the contract, especially at IMPs, but even at matchpoints.

However, if both clubs have been thrown, then you can safely lead a spade and be sure of an overtrick.

Principle #3 Winners are valuable to save because they restrict declarer's options.

Principle #4. In a fuzzy squeeze, it is usually to your benefit to know how many of each suit are remaining.

If there was one club out, then it looks safe to lead a spade. The worst that can happen is that they cash a club. However, the hidden problem is that you might be caught in a pseudo squeeze. Do you throw your winning diamond or your winning spade?

One answer can be had from counting the spades. (Principle 4) If there is one spade out, then there is no danger, you will know to save the ace of diamonds. If there are two spades out, then one of two things happens when you lead a spade. If they both follow, then all the spades are gone and you can safely save your ace of diamonds. If RHO shows out, then after LHO wins the spade and cashes a club, LHO's last card has to be a spade.