Consider the following situation. You hold:
and your partner opens 1NT. You bid 2, intending to raise partner's 2 to 4, but instead the auction proceeds as follows:
Result: 4 was cold, and 4 goes down only one. Bidding 4 originally would have made it harder for the opponents to enter the auction, but then you would have been playing the contract, with your partner (the 1NT opener) as dummy.
Texas transfers are a convention designed to deal with this problem. Here's how they work: after a 1NT opening, 4 and 4 are transfers to major-suit game:
The tranfer shows only that you want to be in game. Normally, you should have game-forcing strength with a 6-card suit to transfer, but you could also transfer preemptively with a weak hand.
Texas in Competition. You may also use Texas transfers in competition. This allows you to jump after an overcall while still arranging for the strong hand to be declarer:
Mild Slam Tries. Playing Texas transfers, responder should always use the sequence:
|1NT 4 4|
to signoff in major-suit game. The sequence:
|1NT 2 2 - 4|
becomes a mild slam try. Opener should usually pass, but may explore slam with spade support and a maximum hand.
Blackwood After a Transfer Since 4 is Gerber after a 1NT opening, there is normally no reason to want to transfer and then use Blackwood. Hence, the sequence:
|1NT 2 2 4NT|
is generally used to show 5 hearts, and invite slam in hearts or notrump (similar to a 2NT bid, but for slam).
However, the situation becomes far less clear when you are playing Keycard Blackwood. In this case, you might want to transfer and then bid Blackwood so that you can find out about the king and queen of trump. Playing Texas transfers, you can have your cake and eat it too: