This convention by Bill Cole and Lynn Deas was designed to show some awkward hand patterns and strengths. Opener re-bids 2C after a 1C or 1D opening and a major suit response. This is forcing and may include various hands. The most common and helpful sequence is:

Opener has shown exactly 3 card support for spades, whereas a direct raise would have been 4-card support with 15-17 points. A re-bid of 3S would be 4-card support, but not quite enough strength to jump immediately.