If you were allowed to look at your partner’s cards and bid first, what would you do?  The optimal strategy would be to figure out the best possible spot and bid it immediately, taking room away from the opponents so that they could not figure out whether you were bidding to make it or to sacrifice and whether they should double you or whether they could get a higher score by bidding to their own game or part score.

A weak 2 opening bid shows a six-card suit and a hand (5-10HCP) that is too weak to open at the one level. We limit it to 10 HCP, because, when you have 11 HCP and a 6-card suit, you will be at least 6-3 shape with 11 HCP passing the Rule of 20, which is an opening hand.  The Rule of 20 refers to a modern opening hand valuation method where you add the number of cards held in the two longest suits to the number of HCP.  If they total 20 or more, you open the hand 1 of a suit.

Many players have such rigid rules for opening weak 2-bids that they almost rarely open them.  Weak 2-bids work because they get in the opponents’ way, not because your hand looks perfect for a weak 2.  So, find excuses to open weak 2-bids not excuses to pass them.  Don’t let voids or outside aces stop you from opening a weak 2-bid.  There should only be 2 things to worry about: 1) a reasonable suit; and no 4-card or longer major on the side.  We don’t open with a 4-card major on the side, because we do not want to miss playing that major if that is our fit; and we don’t want partner to think the opponents have that major as a fit.  As for suit quality, most people were taught to have 2 of the top 3 honors.  Again, preempts work because they get in the opponents’ way.  You should agree with your favorite partner about how aggressive you want to get, but certainly 3 of the top 5 honors is acceptable, and you can be even more aggressive if you both agree.  Please remember that in standard bidding, a 2♣ opening is not weak; it shows a strong hand. So if you have a six card suit in clubs open 3 ♣.

2-level preempt in anything but clubs (2♦ 2♥, 2♠) shows a 6-card suit (5-10HCP)
3-level preempt shows a 7-card suit (5-9HCP)
4-level preempt shows a 8-card suit(5-8HCP)
5-level preempt shows a 9-card suit (usually a minor)

A weak two-bid serves two important purposes. It can be constructive and it can be obstructive.
By making a weak two-bid, you are describing your hand to help partner. This helps partner decide how high to bid and what  suit to compete in. It can also help partner know what to lead if you defend.
A second purpose of a weak two is to interfere with the opponents’ bidding. Most pairs can bid to a reasonable contract if left to their own devices. Bu,t what if you bid in front of them? What if you not only bid, but the bid is at the two level? This can present awkward problems, even for experts, which may leave your opponents guessing.

Many of you may be familiar with the Law of Total Tricks which says that you will get the best result available on a hand by contracting for the number of tricks equal to the number of trumps your side has (the 4-level if you have 10 trumps, etc.)  Your advantage is that you know where your fit lies; the opponents have yet to find theirs.   By raising partner’s suit you make it that much harder for your opponents to uncover their fit. Avoid “slow rolling”.  You must immediately raise partner’s preempts to the law of total trumps. It is a huge mistake to pass and later raise partner after the opponents have had a chance to bid.  Take their bidding space away from them immediately.
The more trumps, the higher you should raise opener’s weak 2 bid.
   - with 3 trump: bid 3,  
   - with 4 trump: bid 4,  
   - with 5 trump: bid at least 4, maybe 5

A few guidelines for responding to preempts:
   • Partner doesn't need much trump support from you (only 2). Preemptor’s suit is usually your best trump suit.
   • You need around sixteen points to consider a game after a preempt. When partner preempts, appreciate aces and kings and devalue scattered high cards. Think in terms of tricks you might lose opposite the hand partner's preempt described.
   • A new suit below game bid in response to partner's preempt is forcing. It shows a good suit of at least five cards and enough strength to consider game but preference for playing in your own suit.

Experts virtually universally agree that new suits in response to partner’s preempt should be forcing and show good hands of around 16+ points if we don’t have a fit with partner. A new suit is usually a good five+ card suit, looking for the best game
With no support, a bad (or medium) hand and a suit of your own, just pass!  Partner rarely has more help for your suit than you do for theirs, and bidding a new suit forces you to higher levels. If your hand is weak, your opponents will often rescue you by bidding.

2NT is rarely used since responder usually passes with no fit and raises with a fit. To bid 2NT you need a good hand (usually with a fit for partner's suit), at least invitational to game. 2NT asks opener to show an outside Ace or King by bidding that suit at the 3-level. If she doesn't have one, she "retreats" to 3 of her suit.  Usually, you'll need this to know if you can bid 3NT. If opener doesn't have a stopper, she rebids her suit. Some people play that you only show a feature with a maximum preempt; discuss this with your partner.
    • 3C: Club stopper.
    • 3D: Diamond stopper.
    • 3H (after 2S opening): Heart stopper.
    • 3S (after 2H opening): Spade stopper.
    • 3(of the same suit as opening): no outside stoppers or minimum (5-7HCP).
    • 3NT maximum but no outside feature (8-10HCP).

When the opponents open, a jump overcall by our side shows the same as it would if you were first to open:
2-level = 6-card suit.
3-level = 7-card suit (may be 6 if you cannot jump to the two level).
4-level = 8-card minor or 8-card or longer major.
5-level = 9-card or long suit in a minor.

If you have a preemptive hand, don’t overcall on the1 level. You should bid at the two level because it is both more descriptive and it takes an extra level of bidding away from the opponents.  It is more descriptive because rather than showing 5+ cards and 8-17 HCP, the jump overcall shows exactly 6 cards and less than an opening hand.  A non-jump overcall on level 2 shows 12-17points and a 5+ card suit.