Wrong play
If you play the ♥5 to one of Dummy's Honour cards first, then return to your ♥Q you will be cut off from Dummy, unable to cash the remaining three heart winners.
Correct play
Play the ♥Q from Declarer's hand first, then cross over with Declarer's ♥5 to Dummy. Dummy wins the trick and can now run off all remaining heart winners.
Another typical card combination is :
Declarer
♦ A Q J
| opposite
| Dummy
3 5 9 10 K ♦
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Play the ♦A and ♦Q in Declarer's hand first. Then lead the ♦J and take it over with Dummy's ♦K. Dummy now has the lead and can play out the two remaining diamond winners.
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