Conventions in depth
A 2♣ Stayman bid on a weak hand with both four-card majors and short clubs, made to escape 1NT.
Responder holds exactly four hearts and four spades with at most a singleton club — and therefore at least three diamonds — and fewer than 8 points, far too little to invite game. (A five-card major would have been transferred instead.) Left in 1NT with a club singleton, the hand rates to play badly; Garbage Stayman offers a way out.
Responder bids 2♣ intending to pass whatever opener replies. Every answer (2♦, 2♥ or 2♠) plays better than 1NT.
When opener has no four-card major and bids 2♦, responder is happy to sit for it: there is a real diamond fit underneath. That is exactly what separates this hand from the related short-diamond hand.
The partner convention, for a weak hand that is short in diamonds and so cannot pass a 2♦ reply, is Crawling Stayman.
