Basicly, the defence has no link with the JAZZ system as such. One can play JAZZ16 but keep the defence that they are used to. As an active bridge player you spend on the average 2/3 of your active playtime defending. So it is worthwhile to have clear agreements on this facet of the bridge sport. On these pages we layout our road of defence.
Intervening the auction started by the opponents can have several good reasons :
presenting an good lead for partner when the opponents must play the final contract
trying to find a contract to play
disturb the opponents in finding their optimal contract
These goals depend on the level of the opening : if opened on level 1 (eg.1NT) the target will be to disturb, while after a preemptive opening we will try to find our contract to play.
Double (Two Way)
Obviously, a double will only serve the second goal and therefore is 'take-out'.
It promises the other suits with a 3 or 4 card and 12-15hp.
If however, partner chooses one of the remaining suits and yet, the 1st defender switches from suits or bids NT, he will have 16hp.
Against an announced transfer
When opponents do a transfer, there is suddenly an opportunity to send useful information towards partner :
if you double the suit used to announce the transfer, you have values in that suit (eg.preparing a lead for partner)
bidding the suit of the transfer is like a take-out double w.r.t. that suit