Lay that pistol down
If someone is aiming a gun at your head, it is a good idea — as a matter of self-preservation — to disarm him if you can safely do so.
An analogous situation arises at the bridge table when a particular opponent threatens your chances of making the contract. In such a circumstance, you should do whatever you can to eliminate the threat.
Take this case where you are in three notrump and West leads a spade. You win East’s jack with the queen, and it is obvious that the contract is ice-cold if West has the king of diamonds. In that case, repeated finesses will overcome the king, and you will finish with 11 or 12 tricks.
But if you start out by attacking diamonds and East has the king, there is a real danger that a spade return will ultimately do you in. This could easily happen if West started with five or six spades and the ace of hearts.
Once you recognize the threat posed by West, it is not difficult to find the appropriate countermeasure. So at trick two you lead the queen or jack of hearts instead of attacking diamonds. This play virtually eliminates all chance of going down.
If West has the ace and takes it, you plan to duck his spade return and win the next one. Now when you take the diamond finesse, you are on safe ground. If the finesse loses, either East will not have a spade to lead (because West started with at least five of them), or East will have a spade to lead (because the spades were originally divided 4-4).
Similarly, if East has the ace of hearts and takes it, you duck his spade return before trying the diamond finesse. Either way, the contract is secured by leading a heart first, effectively disarming West by removing the pistol — the ace of hearts — from his possession before he has a chance to use it against you.