Subtract the card led from 11 and that tells you how many cards are higher then that card in the other 3 hands. When partner leads the 5 (11-5=6) there are 6 outstanding cards higher then the 5. As a defender you can use partners lead to help choose which card you should play.
Partner leads the 5 – Dummy has 2, 6, 10, K and you hold 4, 8, J, A. Declarer calls for the 2 from dummy, what card do you play?
Using the rule of 11 declarer has no cards higher then partner’s 5 since dummy has 3 higher and you have 3 higher. Partner’s 4th best lead was 5, 7, 9, Q. Play low, partner’s 5 is high. Declarer deserves no tricks with this holding.
If you are interested, here are all the previous Tips of the Week