A conventional response to opening bids of 1♥ or 1♠, showing game values or better and primary trump support, now popular with tournament players. A common system of opener’s rebids is as follows:
3♣/3♦/3 of unbid major singleton in suit bid.
3 of agreed major waiting, unsuitable for other actions.
3NT only four cards in the major.
4 of new suit pronounced two-suiter (good five-card suit on the side).
4 agreed suit minimum, no slam interest.
The two big advantages of the method are: You can find out a singleton in opener’s hand (like a splinter). By agreeing the trump suit at a low level and creating a game-forcing situation, the partners can exchange cue bids without having to go beyond game.
Without the information that West is short in spades, meaning that the hands fit very well, it would be hard for East to envisage a slam.