First be clear about this: bidding the fourth suit has ALWAYS been forcing.

In this sequence:
    1    1♠
    2♣    2
the 2 bid is a new suit by Responder and Opener must absolutely bid again.
In standard bidding that 2 bid would be showing a suit, probably in hopes that Opener would be able to bid Notrump.

But when you think about it, if Responder actually held s he could have just bid Notrump himself, so in this sense the bid is not very useful.

Using Fourth Suit Forcing Convention the bid of the fourth suit takes on a new, artificial, meaning. It now says: "Partner, I am strong enough to make a bid but don't have anything I can say, so I'm just bidding the fourth suit. I want you to describe your hand further."

Assume that with the bidding sequence above you held this hand:
   ♠ A Q 9 8 4   K 3 10 7 3 ♣ A 10 4

You cannot rebid your ♠ suit without 6 of them, you cannot support s with only 2, you cannot support ♣s with only 3 and you cannot bid Notrump without a stopper. So what are you going to bid with your 14 points? Of course, you bid 2, Fourth Suit Forcing.

Opener has certain priorities to follow after partner has bid the fourth suit.
  • support Responder's first suit with 3 of the suit
  • bid Notrump if she has a stopper in the fourth suit
  • find some other bid if she cannot do either of the above
Lesson available.