RMB1, on 2011-November-02, 09:06, said:
I don't think that North on his own can have a concealed partnership agreement.…
As I understand it, there has to be some apparent conspiracy between both members of the partnership to bid one way and explain it another way for there to be a fielded misbid ruling. If the bidder thought he was bidding according to partnership agreements (and those agreements are permitted) then it can not be "fielded". It can be deliberately misexplained and perhaps there should be an automatic penalty for that.
I don't see how there need be any conspiracy. That would require South to
deliberately misbid,
knowing his partner will field it. Who does that?
I think North can field a misbid if he's aware from partnership experience that his partner frequently misbids in this situation, even if his partner is completely unaware that he does it. Law 40C1 says, in part "Repeated deviations lead to implicit understandings, which then form part of the partnership’s methods and must be disclosed in accordance with the regulations governing disclosure of system. If the director judges there is undisclosed knowledge that has damaged the opponents, he shall adjust the score and may award a procedural penalty." There's nothing here that implies that South (in this case) must be aware of his tendency to misbid. IOW, it's an understanding, even if both players in the partnership don't understand it.

Of course, even if they don't discuss it, if North starts alerting and explaining properly, South will soon be clued in.
I had a novice partner once who did not "get" Stayman. She would open 1NT, I would bid 2
♣, she would bid 3
♣. We talked about it, I explained that "everybody" plays Stayman, and why, she agreed to play it that way, and then she continued to misbid at the table. When I started alerting her 3
♣, and explaining it as "she thinks I have clubs" or "she forgets that 2
♣ is Stayman", she got upset, saying that I was embarrassing her. So we took Stayman off the card, and I had to remember that 2
♣ was natural. Boy, that was hard!