S/EW		♠	K J 10 3
		♥	A 10 3
		♦	9 3
		♣	Q 10 9 7
♠	8 2
♥	Q J 5
♦	K 10 8 7 2
♣	A 3 2

IMP-scoring (team match), so West will do his utmost to defeat the contract. Overtricks (or extra undertricks) are relatively unimportant.

West leads the ♦2. East wins with the ♦A and continues with the ♦J, South following suit with the ♦4 and the ♦5.
Since dummy's ♦9 now disappears, West's ♦1087 become equals. South still has the ♦Q of course.
If West now plays a small diamond, he will succeed if East has another diamond (very well possible, since from originally ♦AJx he will return the middle card, the ♦J). EW will thus score five diamond tricks and the ♣A: down two.
But what if East started with the bare ♦AJ?
How should West defend?

Solution

Counting is the thing. West can see that declarer at best can make four spade tricks and two heart tricks without surrendering the lead. So without club tricks or extra heart tricks he cannot make nine tricks. This means that West can afford to give South a diamond trick, which will only be declarer's seventh trick. So West overtakes East's ♦J with the ♦K and continues the suit. South is welcome to score the ♦Q. This is not a waste...

S/EW		♠	K J 10 3
		♥	A 10 3
		♦	9 3
		♣	Q 10 9 7
♠	8 2			♠	7 6 5 4
♥	Q J 5			♥	9 8 6 4 2
♦	K 10 8 7 2			♦	A J
♣	A 3 2			♣	8 5
		♠	A Q 9
		♥	K 7
		♦	Q 6 5 4
		♣	K J 6 4

...since West is bound to regain the lead in time — either with the ♣A or an unlikely heart — in order to cash two more diamonds: safely down one.
Admittedly: if East has a third diamond (unlike in the given layout), West's line of defence will cost an undertrick.
But if, in the given layout, West fails to overtake the ♦J, declarer easily makes his contract. So overtaking is in fact a safety-play in defence.