The basic idea is to use transfers in order to give the overcaller a chance to bid twice with very good hands. Initial actions are:
|Dbl||Good 14+, usually balanced|
|2||Transfer to hearts|
|2||Transfer to spades|
|2||Spades and a minor|
|2NT||Hearts and a minor|
Note that initial actions are sound overcalls. Most overcalls are something that resembles a decent opening bid with some playing strength. This means that we might get shut out with 13 vs. 13, but since it is quite possible that we have a game, we can't afford to bid light and get doubled at the 3-level after a game try.
If they redouble for business, pass is to play. System should definitely be off.
We play that after a double, the next double is takeout, usually showing exactly two cards in their suit. Others prefer that the first double is only semi-natural and just shows extra values. Discuss this with partner.
We play that after 1NT is doubled, we are in a force through 2. Some prefer to make this 2, because the force on 2 usually leads to our contract of 2, whereas being forced vs. 2 forces us to the three-level.
In response to 2, 2 shows no or little preference. This is quite helpful if overcaller is 5-4. 2NT is a natural game try. Jumps are invitational. 3 is to play. 2 followed by 3 is forcing.
In response to the transfers, all bad hands simply accept the transfer. New suits are natural and forward-going, but not 100% forcing. 2NT is a natural game try. Jump acceptances are invitational. Yes, there is no force. That probably ought to get changed.
In response to 2 showing spades and a minor, 2NT is an artificial game try, asking for size and the minor. 3 is pass or correct. 3 is invitational.
Over 2NT, showing hearts and a minor, 3 is
pass or correct, 3 is natural and forward-going,