Look for a side suit in the dummy that has shortness — zero, one, or two cards (a void, singleton, or doubleton) — and then check to see whether you have more cards in that suit than the dummy. When you find such an unequally divided suit, you’re in business.
This hand shows how you can open up shop. Assume that hearts is your trump suit and that the spade suit is a side suit. Pretend that the dummy has three hearts.
Don’t attempt to trump a loser in the wrong hand — in this case, the long trump hand.
The spades in in this hand meet the criteria; you see two spades in the dummy and longer spades in your hand, an unequally divided suit. As it stands, you have three losing spades in your hand. However, if you lead spades twice and concede two spade tricks, the dummy won’t have any more spades. When the dummy is void in spades, you can trump your remaining spade loser from your hand in the dummy. After all, losing two spade tricks is better than losing three spade tricks.