From another nice guide by Tyler Wong:
You should lead trump if:
- You don't have a good non-trump lead (maybe your RHO is out of the suits you hold).
- You want to run out trump. Running out trump can be a good tactic if you suspect your opponents are shorter and/or have worse trump than you and your partner (perhaps they both have trumped earlier in the round). By running out trump, you turn the round into a battle of non-trump high cards, so be sure that you have high non-trump, or that your opponents are out of the suits you own.
You can run out trump a couple of ways:
Start from the top and lead down. This is effective if you have a few boss trump, or are very long in spades (six or more).
You lead low trump, your partner plays as high as possible (if partner's highest spade is higher than your LHO's spade). If your partner gets the lead back, she leads low, and you play high. This will work even if your opponents have higher trump than you, as long as you end up with the last trump and can lead suits in which your opponents are weak or void.
Take this hand:
♠AQJ954 ♥QJT97 ♣9 ♦5
Without the big trump power, you would count none of your hearts as tricks. But in this hand, if you draw out the ♥A and ♥K then run out trump, the rest of your hearts will be boss and will be worth two to four tricks.
- Once you suspect your opponents are out of spades, stop leading them.