This mechanism allows the partnership to use the 1NT response for a greater variety of hands: in particular, invitational as well as minimum responder holdings.
The forcing 1NT bid shows 6 to 12 HCP, denies the ability to make a single raise (but not necessarily an invitational raise), and denies holding four spades if the opening bid was 1♥; it must be announced as "forcing" by partner.
Some partnerships agree not to rebid a two-card suit, preferring to promise a minimum of three cards.
The forcing next step variation (discussed below) overcomes these problems by allowing opener to show a balanced hand, or any second suit and guarantee four cards in it, yet still offer the option of playing in two of the major.
With a singleton or void in responder's suit, opener should rebid her original major.
In line with the above Variation, when responder bids their own suit it is 'drop-dead', and Pass should be the expected response.
Playing the forcing notrump, however, it is sometimes tactically advantageous to bid 1NT with this hand and then correct to 2 of opener's major.
This keeps the bidding alive and allows partner to take another call even though your preference would in fact be for the minor.
In standard forcing notrump, responder with five spades bids 1♠ over opener's 1♥ but if she also has 2 hearts she does not then know whether to rebid 2♥ or 2♠.
In forcing next step, opener will raise 1♥ - 1NT to 2♠ if she has support, so if she makes another bid, responder can happily give preference to 2♥.
Here is a more complex approach: Over a sequence of 1♠ - 1NT, opener rebids: If responder wishes to know further about the 2♣ bid, she asks with 2♦.
Similarly with a sequence starting 1♥ - 1♠, opener rebids: If responder wishes to know further about the 1NT bid, she asks with 2♣.