Source: ABF youth bulletins
Take a look at this problem:
Jumping to 3NT may be your first instinct, but this is a bit premature. Imagine if partner has these type of hands:
(a) 
Kx 
Jxx 
xx 
AQJxxx Opposite this minimum hand, 5 is a much better contract.
(b) Kx xxx Ax AQxxxx Opposite this hand, 3NT is cold but 6 is also cold!
Essentially, with so many different possibilities, we must at least try and see whether 5 or even 6 is a possibility before we commit to 3NT. How do we do this? A very well-known convention, also known as the Bourke Relay, will help you out. When you have an uncontested auction of 1minor – 1Major – 2minor, use the next step bid as artificial and game-forcing. So in the auction above, that would be 2 . This would allow you and your partner to continue exchanging information about your hands until you both decide what the best contract is.
For example, imagine after rebidding 2, your partner now rebids 2, suggesting four hearts. You can now bid 3 (cannot be passed out as we are in a game-forcing auction!) and your partner now rebids 3. What do you know? Your partner must be something like a 2=4=1=6 shape, and now you have the information that 3NT is likely down on a diamond lead! Not only that, 6 is starting to look like a real possibility!