In contract bridge, an underruff is a play in which declarer ruffs a trick and a defender deliberately ruffs with a lower-ranking trump. This is a counter-intuitive maneuver which, on rare occasions, may offer some critical benefit to the defense.
Example
The late writer Alan Truscott (1925 - 2005) wrote several articles about the underruff for The New York Times. In this deal, Italian world champion Benito Bianchi declared 6
and received the opening lead of the 
9:1
| | North | | |
|  AQ105 | |
| 84 | |
| West | AJ8642 | East |
| K94 |  7 | 8732 |
| J962 | | 53 |
| 93 | South | K107 |
| KJ42 | J6 | Q863 |
| AKQ107 | |
| Q5 | |
| A1095 | |
"Bianchi won the diamond lead with the ace in dummy, entered his hand with a club to the ace and led the J. West was forced to cover with the king, and South won with the ace and continued the suit. On the third round he discarded the Q, and was able to ruff a diamond and ruff a club. He then ruffed a diamond with the Q and ruffed another club to reach this position:
| | North | | |
| 5 | |
| — | |
| West | J86 | East |
| — | — | 8 |
| J962 | | 53 |
| — | South | — |
| — | — | Q |
| AK10 | |
| — | |
| 10 | |
"Playing dummy's 5 and throwing the 10 now did the trick. West was forced to ruff his partner's winner and lead from the J.
"West had missed the chance for a very remarkable defense indeed. Instead of discarding a club when the third round of diamonds was ruffed he should have underruffed with the 2. He should then have unblocked his K, leading to this ending:
| | North | | |
| 5 | |
| — | |
| West | J86 | East |
| — | — | 8 |
| J96 | | 53 |
| — | South | — |
| J | — | Q |
| AK10 | |
| — | |
| 10 | |
"The J is led and East ruffs. South ruffs with the K, and West must pass the final test. If he underruffs for the second time, he defeats the slam."
References
1Truscott, A. (1985, August 4). An Underruff Defeats a Slam.
The New York Times.
