Scoring: MP
ACBL club matchpoints game.
South passed as dealer (!) and North opened 1♦ in third seat.
East overcalled 2♥ and South undercalled 1♥ and then substituted a correction of 3♥ after realizing that 1♥ was insufficient. The TD (me) was summoned.
I took the South player away from the table and discovered several fascinating and unexpected things:
--he downgraded his club honours and chose not to open as dealer
--he was trying to respond 1♥, having not seen the overcall
--a bid of 3♥ in this pair's system actually shows a near-opener with hearts and is intended to counter psyches
Just to be sure, I took the North player away from the table and confirmed that 3♥ would show hearts.
West was not interested in accepting 1♥, so I ruled that 3♥ was a legal substitution, not conventional, nobody barred--but please wait for the TD next time.
The N-S pair now got to 6NT for a top, against six 920s (6♦), a 420 (5♦+1), a -100 (7♦*-1!), and a 500 (1♥*-2!!!)
Have N-S gained through the insufficient bid or is it the overcall and their unusual agreements that got them to the best spot?

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