The Come-off signal  by Steen Muller (Denmark)
                   
N/S Vul. Dealer North.

                           S A87
                           H A873
                           D KQ8
                           C KT6
                 S QT953             S K6
                 H Q654              H KJT9
                 D 76                D AJT4
                 C 32                C A74
                           S J42
                           H 2
                           D 9532
                           C QJ985

          WEST        NORTH      EAST     SOUTH
         Huyland  Christiansen   Bakke   Blakset
                       1NT        Dbl       2C
           2S          Pass       Pass     Pass

The 2C bid was part of the Danes' escape system and showed both minors. North led the DK. The DA won, South discarding the nine, and the declarer immediately returned the DJ from dummy, setting up a discard for his losing club.
Now the Danish defenders had only three top tricks, and they needed three ruffs to beat the contract. As hearts is the only suit in which to get ruffs, the chance seems remote, but nevertheless the defense managed three (!) ruffs in hearts.
Winning the DQ, North obeyed partner's come-on signal with the D9 and played the HA and another heart. South ruffed and returned a club to dummy's ace. The losing club was thrown on the D10, and East ruffed a club to play a spade towards the King. North went up with the Ace and gave South another heart ruff. The winning D5 forced West to ruff high, and North got rid of his last heart.
Unfortunately for West, the spade suit was blocked, and when East played a spade to the King, he saw that the spades were 3-3 and that he could not return to his hand by ruffing a club, as this would cost him his last trump, setting up North's last spade as a winner. He therefore tried to sneak back with a heart to the Queen, having unblocked the King at an earlier stage, but North was able to ruff, and in this way N/S succeeded in getting their third heart ruff.
At the other table, the Danish West managed to make 3H after a fifteen minute struggle, and Denmark gained 5 imps in the hard fought Scandinavian derby.

Now you may ask, what a Come-off signal is. The convention was invented about forty years ago by the late Danish international player and system-developer, John Trelde. The convention has, of course, a Danish name, and when I explained the convention to Barry Rigal, he immediately named it the Come-off signal.
It is combined with the use of upside-down signals and normally works this way: When you and your partner have a lot of honors in a suit, that your side is known to hold, the discarding??? of a honor (including the nine and sometimes the eight) is commanding your partner to switch. An even honor asks for the lowest suit and an odd for the highest suit.
Try this lay-out:
                                  East
                                 S  752
                                 H  QJT
                                 D  QJ
                South            C  KQJ87
               S AQJ64
               H K42
               D K7532
               C -

                 WEST   NORTH   EAST  SOUTH
                  1H     Pass    2C     2S
                 Pass    Pass    3H    Pass

North led the SK, and now it is easy enough the hold the declarer to nine tricks. You overtake the SK with the ace, cash the queen and force partner to ruff the S4 to request him to play a club for you to ruff.
Unfortunately, that won't beat 3H, but the Come-off signal will. You play the HQ under partner's king, commanding a switch to clubs - not just suggesting a switch.  You ruff the club, cash a spade and let your partner ruff a spade to give you one more club ruff.