The Under Jump Shift is used to show
(a) four card support for opener's major
(b) 9+ to 12- HCP (limit raise values)
(c) a singleton or void in a side suit
Reponder does this by making a jump shift in the suit immediately below opener's major (i.e., 3D for a 1H opening, 3H for a 3S opening). Opener interested in slam may then ask for the shortness by bidding three of the major suit, or sign off by bidding game directly (one may not signoff in three of the major because that is the asking bid; this should not be a concern because the fit plus side shortness makes game extremely likely).
In response to the asking bid, responder bids as follows
Where opener bid hearts and bid 3H as the asking bid (i.e., 1H pass 3D pass 3H!)
(i) 4C or 4D = singleton club or diamond
(ii) 4H = singleton SPADE (you'll see why next)
(iii) 3S = shortness is a VOID
OK, after 3S showing a void, opener may bid 3NT to ask its location, to which responder replies
(1) 4C or 4D = void in clubs or diamonds
(2) 4H = void in spades
Can you see the benefit here? Without losing any room, opener can learn whether responder has a singletone or void, and where it's located WITHOUT ever losing the ability to signoff in game.
Where opener bid spades and bid 3S as the asking bid (i.e., 1S pass 3H pass 3S)
(i) 4C, 4D, 4H = singleton club, diamond, or heart
(ii) 3NT = shortness is a VOID
Here we go again. After 3NT showing a void, opener may bid 4C to ask its location, to which responder replies
(1) 4D, 4H = void in diamonds, hearts
(2) 4S = void in clubs
Again, notice that opener never loses the ability to signoff in game, yet can learn whether the shortness is a singleton or void, and where it's located.
Thie following explanation comes from Greg Morse, gmorse @mmtek.com
Opener
1H-3D* UJS
3H Slam interest (17 HCP and up. Or use the losing trick count. Play responder for 7 losers.) Where are you
short?
Responder
-4C/4D Short clubs or Diamonds
4H Short spades!!!! Note. 4H/P Shortness not useful for slam. Sign-off
3S My shortness is a void.
Opener
3NT Where? (This is a repeat of the asking bid)
Responder
4C/4D here.
4H Spades.
Opener
4H/P Shortness not useful for slam.
Note that opener can find the shortness without going past game if he
wants, or just bid game leaving the opps in the dark as to shortness (excellent point!)
location.
Here are a couple of examples
North
AQJxx
AJ
KQxx
xx
South
Kxxx
Kxxx
Axxx
x
North East South West
1S P 3H(1) P
3S (2) P 4C(1) P
4NT P 5H P
6S Pass out
(1) showing limit raise and side shortness
(2) asking location of side shortnes
(3) showing singleton club (if void, responder would have bid 3S)
Using Roman Keycard Blackwood, the partnership finds the 27 HCP slam.
North
AQJxx
AJ
KQxx
xx
South
Kxxx
KQxx
x
Kxxx
North East South West
1S P 3H(1) P
3S (2) P 4D(1) P
4S Pass out
(1) Under Jump Shift
(2) asking bid
(3) shows shortness in diamonds causing opener to signoff in game.
One more..
North
xxx
KQJxxx
AKx
x
South
-----
Axxx
QJxx
Axxxx
North East South West
1H P 3D(1) P
3H(2) P 3S(3) P
3NT (4) P 4H(5) P
4NT (6) P 5H P
7S
(1) Under Jump Shift
(2) Asking bid
(3) Showing that the shortness is a void
(4) Asking location of the void
(5) Showing the void is in spades
(6) Asking for aces (note: because opener knows where the void is located, there can be no confusion when using Blackwood or Key Cards asking bids)
Finding the 24 HCP grand slam.