A convention invented by the English player, Jeremy Flint, designed to allow the partnership to play in three of a major after an opening bid of 2NT. Immediate responses of 3♥ and 3♠ are natural and forcing, promising at least five cards but 3♦ is an artificial request for opener to bid 3♥.
With long hearts and a weak hand, responder then passes; with long spades, he converts to 3♠. A development of the convention allows the partnership to play in 4♣ or 4♦ (responder converts 3♥ to 4♣ or 4♦).
Nowadays, transfer bids have rendered Flint almost obsolete.