According to the rules of most games, a player must have no cards left in the suit led in order to ruff.
Since the other players are constrained to follow suit if they can, even a low trump can win a trick.
In some games, like Pinochle and Preferans, the player who cannot follow suit is required to ruff.
"Ruff" is normally a verb, meaning "to play a trump card when a non-trump suit was led".
This gives declarer the option of discarding a losing card from one hand while playing a trump from the other, usually garnering an additional trick in the process.
Some indicators that a hand may lend itself to dummy reversal are: W EEast is in 4♠ and receives a trump lead.
At its simplest a ruffing finesse involves a high sequence (e.g. KQJ) opposite a void in the same suit.
Repetition of this will avoid the loss of a trick to the ace, but it only works when the ace is on the right side (i.e. 50%) A technique where one side creates an otherwise non-existing trump trick by ruffing high knowing it will be over-ruffed or, in other situations, by refusing to over-ruff.
A play that involves one player ruffing high in the hope that an overruff by an opponent will result in the promotion of a trump card in partner's hand into a winner.
A play involving a simple squeeze where a side suit presents a menace which can be established by ruffing.