Sometimes you have to bid, but your hand doesn't fit any of your options. For example, playing 2/1 Game Force, you open 1♠ holding ♠AQxxx ♥xxx ♦xxx ♣AK. When partner responds with a 2♣ game force, none of your rebid options are attractive. You aren't eager to rebid 2NT without an honor in diamonds or hearts. You aren't eager to rebid 2♣ on a 5-card suit. You don't have a second suit to bid. Passing isn't an option after partner has forced to game. Most pairs playing 2/1 Game Force have an agreement on what to use as their default bid (a punt bid) when your hand doesn't well fit any of your options. If playing a 2/1 style which emphasizes shape, the default rebid is 2NT. A 2NT rebid shows a balanced hand, but does not guarantee stoppers in the unbid suit. If playing a 2/1 style which emphasizes strength, the default rebid is 2 of opener's major. Opener rebid of 2 of their major doesn't show or deny extra length. Rebidding 2NT guarantees stoppers in the unbid suits. Agreeing on a default punt bid cleans up all your other bids. Using 2♦ as the default punt bid is a great way to improve 1♥/♠ - 2♣ auctions. See our 2/1 Game Force video lessons for more on this topic.
The need for a punt bid occurs on many auctions. If you play 4th Suit Forcing to game, the bid of the 4th suit on an auction is an artificial game force. For example, you hold ♠ AKxx ♥ xx ♦ AQxx ♣ xxx and open 1♦. The auction begins as shown below. East's 2♣ rebid is an artificial, game force, 4th Suit Forcing.
West isn't eager to rebid 1NT without a club stopper. Rebidding 2♦ advertises long diamonds. Rebidding 2♥ suggests 3-card heart support. Raising clubs advertises 4+ clubs. Rebidding 2♠ suggests 5+ spades and longer diamonds. West's hand doesn't fit any of these descriptions. If a partnership hasn't agreed on which rebid should be used when the hand doesn't fit any of the options, all rebids are suspect. Serious partnerships with good memories should discuss which bid to use on hands where none of the options fit. Agreeing on your default punt bid cleans up all your other bids.
One possible agreement is using the cheapest available rebid as the punt bid. With that agreement, West would rebid 2♦ on the above auction which would show 5+ diamonds or a hand which needs to make a punt bid. This isn't optimal, but has the benefit of being easy to remember. A better approach is to use the bid which comes up least frequently as the punt bid. Since 6-5 hands are very rare, the above auction would use 2♠ as the punt rebid. All other rebids can be trusted. With a 6-5 hand, West would need to rebid spades again to confirm a 6-5 distribution.