A cue-bid in the opponent's suit in a competitive auction where no major fit is evident, is a request for partner to bid NT if he can 'stop' their suit..
S: AQxx
H: KJx
D: Qx
C: Qxxx
S: Kx
H: xx
D: AKxxx
C: KTxx
W N E S
1D
P 1S 2H 3H!
P 3N P/O
Source: bbuff@user.rose.com
The most important Bidding Conventions