To cover all aspects of preempting would take a large book, so we intend to show some of the critical factors, under the headings - Why, When, Level and Defence.
WHY? The theory is that left to their own devices, players should reach a reasonable contract, though evidence from the travellers sometimes contradicts this. By preempting, whether by opening or by overcalling you have given your side an advantage, because your partner is in a good position to judge who should have the contract, whilst the opponents may well be guessing.
Sometimes of course it backfires, when opponents make inspired bids which they would not have made without the preempt, however you hope to gain in the long run.
Of course, the main reason to preempt the baddies is because it's fun, making interesting scores and stories.
WHEN? Like most things timings and judgment areas are difficult elements, but here are a few guidelines. Remember that the idea is to cause problems for the opponents, not ourselves.
Vulnerability is critical - Green (nonvulnerable v vulnerable), White (NV v NV), Amber (V v V), and Red (V v NV). Also position - whether you are in 1st (ie dealer), 2nd or 3rd position.
Green at 1st position is the safest area, with Red at 2nd the most dangerous - for you are more likely to go for an expensive penalty against little for the opponents, whilst you are equally more likely to preempt your partner rather than the opponents.
With this in mind, Q1098x x J10xx xxx is okay as a Green 1st position weak two, whilst Kxxxxxx Qx Kx Qx would be a horrible second in hand preempt at Red. Suit quality rather than point count is the key factor. KQJ9xxx x xxxx x would be everybody's Red 2nd in hand three level opener.
Also, you are announcing to partner that you only want to play in that suit, which is why KJx Qxxxxx x Qxx is far from ideal as a weak two, since you may want to play in spades, clubs, no trumps, or, if partner has some support, your own suit!
Admittedly some people do agree a completely random style of preempts, but you have got to accept that you may kill off your partner as well as your opponents!
LEVEL? Once you have decided to preempt you also have to judge the level. Basically you want the hardest level for the opponents to judge, without risking a vast penalty yourself. Choose the highest level you can justify to teammates, and then shut up for the rest of the auction.
DEFENCE? There is the `live' position, and the `protective' position. When opponents have opened 3♥ straight in front of you, you are in the live position, and want a decentish opening bid to come in, perhaps 14+ points, though be freer to come in with shortage (add a couple for a singleton in their suit, subtract a point for three card holding). Avoid coming in with a hand suitable for a weak no trump.
The protected position is when they have opened on your left followed by two passes. This is much more tempting as you are less likely to be doubled (doubles by preempter's partner should ALWAYS be for penalties since that person knows pretty accurately what his partner has, and the biggest point of preempting is to take penalties), and partner no longer has a chance to bid.
Some people still do play penalty doubles of preempts, but the fact that the only thing that the top players can agree on is the use of takeout doubles should be sufficient evidence that this really is the best way to defend against preempts. (Some people play penalty doubles because they don't want opponents to `get away with it' - but this is in fact exactly what they do if you play penalty doubles. For, though sometimes you can use the double, far more often you are stuck without any sensible option.)
Whatever you think of preempts, one of the important things to know is what to expect of partner. And that can only come by partnership discussion. So next time you are thinking of preempting, don't count your points. Look at your suit quality, the number of trumps, and what you are aiming to achieve.