Source: CAC 1977 Bulletin
“NEVER FINESSE AGAINST YOUR PARTNER – UNLESS IT’S THE ONLY WAY TO BEAT THE CONTRACT”. by Schmuel Lev
Samuel Lev of Tel Aviv is a member of the Israel national team. They finished 2nd in the 1975 European championships, 3rd in the 1976 Bermuda Bowl, and 8th in the Team Olympiad. This is Lev’s tip in the Bols Bridge Tips Competition.
ONE of the maxims (writes Samuel Lev) which Bridge took over from Whist in its early days was, `Third hand plays high.’ Another slogan that expressed exactly the same idea was, ‘Never finesse against your partner’. Since then, of course, a great deal has been discovered. My Bols tip refers to some quite frequent situations where it can be very good play for third hand to ‘finesse’ against his partner that is, to play the lower of non-touching honours even though dummy has a worthless holding in the suit led.
A common situation occurs in notrumps. It is often vital to winkle out declarer’s stopper on the first round, so that the suit can be cashed when defenders regain the lead:
West leads the 
4 against South’s contract of 3NT. If East puts up the A (“Third hand plays high! “), South will duck the next round of clubs and West’s suit will be dead.
South will be able to develop his ninth trick by taking a heart finesse into the safe hand. But if East plays the J on the first round, it will appear to South that West may have the ace and he will not know that it is safe to duck. South may therefore win with the king and take the heart finesse. Now East continues clubs and beats the contract.
Of course, there is sometimes an element of risk when you finesse against your partner. Here, East may give declarer an unnecessary trick if the holds Q-x-x. But East can afford to take this risk, for he has control of the major suits and can see that the contract will be defeated if West’s club suit can be brought in. East also knows that West cannot possibly have a side entry, and that the play of the jack of clubs is therefore vital. Against a suit contract, a ‘finesse’ may create an entry for a vital switch:
South reaches 4
after North has opened with a Precision 1 and South has subsequently shown three controls — in this case, an ace and a king. West leads 
4. If East makes the normal play of the A, the contract will be made, declarer losing a spade, a heart and a diamond. East can see that a diamond from partner is vital, and he should therefore play J at the first trick, driving out South’s known K. When East comes in with K, he leads a small spade to his partner’s queen. The obvious diamond switch then defeats the contract. A DEFENDER who has bid a suit may often have the opportunity to finesse against partner when this suit is led:
East opens with but South becomes declarer in 4. West leads 2,which suggests that he has an honour in the suit. East therefore finesses the queen, forcing the king. When East comes in with A, it is quite safe to underlead A, as West is expected to have the jack. Now West can find the vital club switch before 
A is dislodged. If East makes the routine play of A at the first trick, the defenders can never make more than their three aces.
[box type=”info”]MY BOLS TIP is this: When you have a holding such as A-Q or A-J in the suit led by partner, do not auto-matically play ‘Third hand high’. By finessing the lower honour, you may sometimes succeed in creating a vital entry to your partner’s hand.[/box]