As a defender, the most important concept you can grasp is: Should we be "safe" or "attack?" Should we be "Passive or Aggressive?"
Passive is leading from three low cards or a safe sequence like 1098.
Aggressive is leading away from an honor or laying down an unsupported ace.
What if dummy is on your left and you consider shifting to a diamond in this situation:
DUMMY   764 |
|
|
YOU |
If you shift to the 2, you are making an aggressive, attacking play. If your partner has the ace, you will collect diamond tricks. Even if partner has the queen, you will be okay.
But, if declarer has the AQ, your play will have cost a trick.
Why make such a play? If the auction, dummy and early play indicate you need to cash out and quickly take (or establish) tricks, your best hope could be a diamond shift.
However, on most deals, you will want to go passive in this situation (and play anything but a diamond).
Say declarer is in 4. If he has diamond losers, (for examples, Q105 or 1093 or even A103), he will lose those diamonds whether or not you shift to the suit. Unless--and it should be a bold unless, he can discard them. If the rest of dummy is "useless"--such as no good long suit or trick-source, declarer will sit there with those diamond losers until he eventually must lose them. If dummy happens to have AKQJ109, then you had better shift to diamonds--or declarer will throw them on dummy's clubs.
Sometimes you will be able to anticipate whether to be passive or aggressive before the dummy even comes down. Listen carefully to the auction. Say the auction (with no interference by our side) goes: 1
--1--2--4 and your hand is:
765 Q82 
KJ72 832
Is it time to be passive or aggressive?
Can't you picture dummy coming down with something like AKJxxx? If so, you better get to work quickly before declarer pitches all of his minor-suit losers on hearts. The best chance to get tricks is a diamond lead.
What about 1--2--3--4? This is a deal where it feels like declarer is going to fight for tricks and you would like to lead passively.
Summary: On defense, usually try to be passive. Don't make attacking/dangerous plays. But, if all the evidence is there that declarer will be able to dispose of his losers if you don't get active, then get active.
Just as a picture is worth 1,000 words, so are examples. Try the four free deals below to see if you can further understand this very important part of defense:
GET LARRY'S VIDEO ON DEFENSE HERE!
For more practice deals, click here, and then download the demo (all the way at the bottom).