With this hand, Graeme illustrates how helpful fourth suit forcing can be when we know we want the auction to continue but we're not yet able to decide our best contract.
When your partner opens 1NT, you know almost exactly what kind of hand he holds. It is one of only 3 possible shapes: 4333, 4432 or 5332 - the balanced hands. And it sits in a very narrow point range: 12-14 if you are playing Acol; or 15-17 if you are playing Standard American.
North opens the bidding 1♦ and East overcalls 1♠. Sitting South, we have a lovely 6-card heart suit but only 6 high card points. We want to do something but aren't strong enough to bid hearts. Luckily, we have the negative double which will ask partner to bid one of the unbid suits. North now bids 2♣ and when it comes back to us, we must decide what to do.