The hand with the greater length in a particular suit, especially the trump suit.
As a rule, ruffing in the long trump hand does not generate extra trump tricks. Nevertheless, you may wish to ruff in the long hand to interrupt the run of an opponent’s suit, to set up dummy’s suit or to prepare the way for a throw in or squeeze.
You must take particular care about ruffing in the long hand if the long hand has only four trumps or if a defender is likely to have four trumps:
As South, you play in 4♠ after East has pre-empted in hearts. West leads ace and another heart. If you ruff, you will go down on the normal 4-2 trump break: West wins the second round of trumps and leads a third round of hearts, reducing your trumps to fewer than his.
Instead, you should throw a diamond at trick two – it is a loser anyway.