Source: 
BridgeBum 
A coup en passant (or elopement play) is a trump promotion play by declarer. Assume that South plays in hearts in this 3-card ending: 
If South is on lead, then all three tricks must be conceded to East. But if dummy is on lead, South can generate a trump trick by leading a spade from the table. If East ruffs, then South’s J is promoted into a winner. If East discards a diamond, then South simply ruffs with the J.
The term “coup en passant” is French for a coup “in passing”. It is also related to the chess play of capturing a pawn “en passant”.
Example
South declares 3. The defense attacks with three rounds of spades, South ruffing the third in dummy. Next comes the A which reveals the bad trump break. The contract is now in jeopardy since South has 5 losers: 2 spades, 2 clubs and 1 heart.
Given the circumstances, South can try for a coup en passant. At trick 5, South can cash the A-K, the A-K, and ruff a diamond in hand. The layout is then:
A club to dummy’s K sets the stage. When dummy’s last diamond is led, East can pitch the 7, letting South ruff. Or East can ruff high, in which case South discards the 8. Either way, South gets a 9th trick. It was important for South to take both heart winners before attempting the elopement. Otherwise, the ending would be:
When South leads a club to the king and plays a diamond off the board, East simply discards a heart. South only gets one top heart before suffering a heart ruff by East. A heart trick must also be conceded to West at the end for down one.
– See more at: http://www.bridgebum.com/coup_en_passant.php#sthash.m9GCDvDZ.dpuf