The “old saw” when in doubt lead trump has been disproven decades ago. However there are conditions that should automatically trigger a trump lead.
When you know these conditions exist, you have to lead a trump regardless of how much it hurts/
CONDITIONS FOR CUTTING DOWN DECLARER’S RUFFING POWER:
1. There is a partial or total misfit –
a. Mere suit preference taken by declarer
b. Your side has a trump stack
c. Misfits in general
2. Bidding suggests dummy shortness
a. Dummy denied notrump due to shortness
b. Dummy bid two suits and raised a third
3. You are strong in the other suits
a. You control the opponent’s side suit
b. You control all other suits
c. Opponent’s are SACRIFICING (STOLE YOUR BID!!!!)
WHEN THE ABOVE CONDITIONS EXIST ——
YOUR BEST LEAD IS A TRUMP!
ANY partial or total misfit is a prime candidate for a trump lead. Misfits that come up most often are mere preferences taken by declarer’s partner in a partscore contract or in auctions that show two suits (ie. Flannery or Michaels) —these auctions practically scream for a trump lead.
When your side has a trump stack (ie. partner leaves in a one level takeout double for penalty by passing) you MUST LEAD TRUMP. Since partner has a trump stack, the only way declarer can make extra tricks is by ruffing in dummy.
When your side controls three suits and the opponents sacrifice, again YOU MUST LEAD TRUMP to cut down declarer’s ruffing power.
Sometimes trump leads are unpredictable and you can still inflict damage by leading a trump once dummy is exposed.