The hand with the fewer number of cards in a suit (usually trumps). Usually it is more productive to take ruffs in the short trump hand than in the long trump hand.
If you are South in 6♥, you can generate an extra trick by giving up a spade and ruffing a spade in dummy, the short hand. This way you make six trump tricks rather than five.
Ruffing in the long trump hand (South) would not be a good idea. If you ruff two clubs, you will have fewer trumps than East and so be unable to draw trumps and cash the diamonds.
