Responses to Interventions After an Initial Pass

Responses to Interventions After an Initial Pass
You are in South and have passed initially

S	O	N	E

You have all the classic responses to your partner's intervention:

• Supports are preemptive, applying the Law of Total Tricks
• Jump shifts are fit-showing bids with a 5-card suit and a 4-card fit
• Cue-bids show a fit and indicate a maximum hand of 10-11 HCP
• The jump cue-bid promises a 4-card fit and a shortage in the opener's suit

And in addition, there are impossible bids that will help you compete when the opener's side continues bidding.

The Impossible 3NT Bid
In response to an intervention after an initial pass, the impossible 3NT bid shows:

• A strong fit in your partner's major
• At least two defensive tricks

For a player who passed initially, the 3NT bid in response to their partner's major bid cannot be natural... easy to understand. After an intervention or even an opening in the third position, the hand can be weak: since the passer themselves has fewer than 12 HCP, they cannot conclude at 3NT!
In this sequence, since the 3NT bid is impossible (and thus available...), it is used to show support equivalent to that of a preemptive raise but with two defensive tricks. In competitive bidding, the partner is well-positioned to find the best contract for the partnership: either a penalty double or continuing the preemptive action, for example.
Observe the two examples below: