♠ AKT73 ♥94 ♦A96 ♣984.
You lead your fourth-best spade, and dummy comes down with
♠ J965 ♥K652 ♦J84 ♣K2.
Declarer plays low from dummy, partner plays the deuce, and declarer wins the eight in hand. Declarer now shoots back the four of spades. What's going on in the spade suit? Well, it seems pretty obvious that partner has a singleton and declarer has Q84. So it seems right to cash two rounds of spades and exit a spade to set up your long card in the suit.
You can use a suit preference signal in this situation to show which suit you want partner to return. Now, if the dummy was something like
♠ J965 ♥AQJT ♦T84 ♣72,
partner would know not to return a heart, so the choice would be between diamonds and clubs. You would cash two spades and continue the ten of spades if your entry was in diamonds, and continue the three of spades if your entry was in clubs. Fairly simple, right?
But in the current situation (dummy is really ♠ J965 ♥K652 ♦J84 ♣K2) there are three suits in play for partner to return if and when he gets in. So you need to go a little deeper and use your spade honors to show suit preference as well! Here's how I would handle this combination:
Ace, then king, then ten (highest card at every opportunity): Please lead hearts!
King, then ace, then three (lowest card at every opportunity): Please lead clubs!
King, then ace, then ten (low then high): Please lead diamonds!
There's also a fourth possibility: ace, then king, then three. My instinct would be to use this to show something in both hearts and clubs but no sure entry.
In order for these subtle suit preference signals to be effective, both partners need to be paying attention throughout the defense- but if you're willing to put in the work, your defense can be devastating.