Jacoby Transfer Convention should only be used if your partnership also uses Stayman.
The basic idea of Jacoby Transfers is that the Responder bids the suit just below his real suit, and the 1NT Opener is 'Transferred' into bidding the real suit. The advantage is that the stronger hand then becomes Declarer and the weaker hand is Dummy.
- 2♦ : tells Opener to bid 2♥
- 2♥ : tells Opener to bid 2♠
The general plan is that with a 5-card Major, Responder transfers partner to that suit, then bids Notrump. This allows Opener to choose Notrump when she happens to have just 2 cards in the suit - and therefore only a 7-card fit.
When Responder has a 6-card suit he transfers partner to that suit, then bids the suit. This informs Opener that the contract will definitely be played in the suit since Responder knows there must be an 8-card fit. Responder is sure of this because the 1NT Opening would not be made with a singleton.