Source: Asociacion Española de Bridge

Problem 1: Dealer: South N/Vulnerable

Q 10 8 7
A Q 10 5 2
J 6 5


A K J 9
A K 9
A 8 6 3 2
West North East South
Paso 1 Paso 1
Paso 2 Paso 4NT
Paso 5* Paso 5**
Paso 5NT*** Paso 6

* 1 de 5 keycards

** Trump Quenn asking

*** I have it, and I dont have Kings

Lead: J

First Trick: J, Q,K,A…how do you play? The fact that you escaped from a trump lead, and that you have all the high trumps, allows you to play a «crossruff» technique: you can make 4 trumps from your hand, and 4 trumps from dummy, plus 4 honors: A, A and K and A for a total of 12 tricks.
However, you must take a simple precaution before starting to ruff: : play first Ay K, to avoid a possible discard of diamonds by a defender, then you wont be able to gain tricks with them.
The correct line is: win with the A, play A and K, and A and then begin the cross ruff.