Source: 
BridgeBum 
A ruffing finesse occurs when declarer holds a sequence in one hand and a void in the other. In this layout, South is on lead with spades as trumps:
South has two natural diamond losers, but can shed them by leading the Q toward dummy.
If West covers with the K, then dummy ruffs. South comes back to hand with the A and discards dummy”s diamonds on the QJ.
If West ducks the Q instead, declarer throws a diamond from dummy and leads another heart, thereby repeating the ruffing finesse.
Example
Figuring dummy for a spade singleton or void on the auction, South optimistically barrels into 6
. West”s opening lead is the 3, won by East”s ace. At trick two, East returns the J.
South knows that East should have played the K (the lower of two touching honors) casino if holding both the ace and king. South can also count 22 HCP between his hand and dummy, and East has just advertised 5 HCP. Therefore, West should hold both the K and K to justify an opening bid.
South, then, should spurn the club finesse and try a ruffing finesse in spades. At trick two, he wins the A and proceeds to draw trumps, finding them 1-1.
South now leads the J through West for a ruffing finesse. If West covers, then dummy ruffs and South returns to his hand via a trump to play spade winners, dumping club losers from dummy. If West ducks the J, then South pitches a club from dummy and continues with the T. Either way, South”s losing clubs can be ruffed in dummy to make the contract. The full deal: