Laurent Chavet submitted this convention. He infoms me that it is very popular in France.

2 clubs after opener has rebid 1nt in reply to a major suit response (e.g., 1C pass 1S pass 1NT) shows that the responder has a five card major and at least 11 HCP. It does not matter that opener may have bid clubs initially. Opener may have opened hearts and received a 1S response. Opener replies as follows:
Ptr opp Ptr Opp
1x pass 1M pass
1NTpass 2C! pass

the answers are then :
2D no fit with 11-12 HCP
2H fit with 11-12 HCP
2S fit with 13-14 HCP
2NT no fit with 13-14 HCP

note :
- the order of 2H and 2S doesn't depend one which major responder initially bid
- the begining can be 1H 1S 1NT

As a contrast, if responder rebids the major, s/he is showing 5+ of that major, but a weak hand (fewer than 10 HCP)

1m pass 1M pass
1NT pass 2M pass

Source: Laurent Chavet chavet@ai.mit.edu