Source: www.qldbridge.com 
One of the mysteries of bridge is when you should draw trumps. Yet the answer is so simple. YOU SHOULD DRAW TRUMPS WHEN YOUR TRUMPS ARE WEAK. When your trumps are weak the great danger is that the defenders will score their big trumps separately. On this deal your trumps couldn’t be weaker:
Dealer: North Vul: E/W
North started with 1, and South ended up declarer in 4. West led J. The weak trump suit is a warning that trumps need to be played ASAP. When declarer played A-K and ruffed a heart, the roof fell in. West over-ruffed, and the defence crossruffed, finishing with five trump tricks. On this deal South declared 4 and West led J.
Declarer won A, and started on a heart-spade crossruff. West ruffed in on the third heart, played A, diamond ruff, and the defence took their four trumps plus A for two down. Just play A and another on winning A, and declarer comes to ten tricks. When your trumps are weak, you just can’t afford to leave those big ones out. When you have strong trumps the time is right to make extra tricks by crossruffing:
South declared 6, on a trump lead. Declarer has all the high trumps, and on a crossruff can count seven spades, A, A-K and A-K (just be sure to cash your outside tricks before crossruffing) Sometimes you need to mix the two strategies. On this next deal South played 4 on J lead.
Declarer won K and ducked a round of trumps. West won 10 and returned K. Ducking one trump meant that declarer could now win A, and crossruff with impunity, with just the master trump outstanding. West could over-ruff, but declarer loses only a diamond and two spades. If declarer had won A and crossruffed, East would have over-ruffed a club, and the defence would make three spades and a diamond.