Rule: When you as a defender have four of declarer’s trumps, you should lead your longest other (non-trump) suit in hopes that declarer is short in that suit.
Since you have five diamonds, the odds are that declarer will be short in that suit. This is called the forcing defense. You will be forcing declarer to ruff in her hand, the hand where usually the most trumps are located, often called the long hand. This will aggravate declarer no end as you, the defender, will end up with more trumps than declarer. It’s not pretty and it is the most brutal defense. It is sometimes referred to as “tapping” or “punching” the declarer.
With all that in mind, West led 
7 versus 4
. Partner won the Ace and returned 10. West won K and returned yet another . Declarer was forced to ruff in her hand. Declarer started to pull trump. She reckoned she would only lose the Ace of trump and the rest was a walk in the park.
West won the first round of trump with A and led another . Oops, now declarer had to ruff a second time in her hand. She led another high and East failed to follow suit. Now West had three left and Declarer only had two. When declarer played 
AKQ, West scored her second trump trick and when the smoke cleared, the result was Down one.
On the hand above, declarer was powerless to get out of this mess. Let’s see what he can do on this next hand.
It is worthy of note that North South were not vulnerable and East West were vulnerable. This is an opportunity for North South to make it tough for East West. South assumes that West has the balance of the points so preempts with his 4
bid making it tougher for West to enter the auction. This bid is sometimes called “the Weak Freak” and shows a bad hand with 5 trumps. As it turns out, if West can make 4, it would have been right for North to continue on to 5 as that contract will only be down 2. Being minus 100 would be way better than being minus 620 if EW is allowed to make 4. Even doubled: Minus 300 is better than minus 620. Let’s see how 4 will fare.
North leads A, followed by K. West ruffs the second lead. Oops. Now his trump suit has been shortened in his hand. If he now draws all the trumps (which break badly this time), he still has to attack his suit which is missing the Ace. When North wins with Ace, he can now “run” his suit and have his way with West. This is ugly. Declarer has “lost control” of the hand because his trump suit is depleted. Even if West delays drawing trumps and attacks his suit sooner, North can duck his A for one round, win the second round of and now give South a ruff.
How could West have saved the day and made his contract. On this hand, West has an automatic loser. On the second lead, he should simply discard a small . This play is called a Loser on a Loser. Now, whatever North leads next, West cannot be hurt as he still has a goodly supply of trumps. Being forced to ruff in the long hand is always troublesome for declarer when he is dealt a normal supply of trumps. As a defender, it is always right to lead high cards in suits which dummy has when declarer is out of that suit. As a declarer, look out; see if you can pitch a loser instead of ruffing, in case you won’t be able to handle the shortened supply of trumps.
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