The Over Jump Shift is used to make a forcing raise of partner's major suit. It is a jump shift in the denomination DIRECTLY above opener's major (i.e., 3S if opener bid hearts, 3nt if opener opened 1 S). The Over Jump Shift shows (a) a good 12 to bad points; and (b) a singleton or void in some side suit.
Where opener has no desire to try for slam, opener simply bids game, thus hiding the location of the singleton from the opponents.
Where opener wishes to search for slam, opener bids the next higher denomation (i.e., 2NT if opened in hearts, 3C if opened in spades). Reponder then bids the suit containing the singleton. If clubs are the location of the singleton (where opener bid 1S, then asked for the singleton with 3C), responder bids three spades. This is similar to the Mathe Asking Bid.
e.g. (provided by gmorse @mmtek.com)
1S-3NT*; Over Jump Shift. 12/15HCP, 4+ spades and Shortness somewhere.
4C Slam interest. Where are you short?
-4D/4H Short Diamonds or Hearts.
- 4S Short Clubs.
4S/P Shortness not useful for slam. Sign-off.
4NT/Cue Slam exploration.
E.g.,
North
AQJxx
xxx
AQx
xx
South
Kxxx
x
KJxx
AKxx
N E S W
1S Pass 3NT(1) P
4C(2) Pass 4H (3) P
5D Pass 6D P
6H Pass out
(1) showing 12+ HCP, four card spade support, and side shortness
(2) asking for the side shortness
(3) shortness is in hearts
Cue bids showing first and second round diamonds controls follow, finding the cold 27 HCP slam (perhaps some would use 4NT to ask for controls before bidding slam).
To show a void responder could rebid hearts at a later point. Note that if the shortness were in clubs, responder would bid 4S.
E.g.,
North
AQJxx
xxx
AQx
xx
South
Kxxx
AKxx
x
Kxxx
N E S W
1S Pass 3NT(1) P
4C(2) Pass 4D (3) P
4S Passout
Slam is unlikely with the wasted values in diamonds (perhaps if responder had the ace of clubs, responder might continue to search for slam on his/her own initiative, though the risk remains high)
(1) showing 12+ HCP, four card spade support, and side shortness
(2) asking for the side shortness
(3) shortness is in diamonds