Rather than 'pick a minor' this is more commonly described as
'scramble'. The theory is (assuming double was take-out) that
- South has defined the strength of their hand, so there is no need to
play (e.g.) 2NT as lebensohl
- A natural 2NT is not very useful, because with good spades South
would pass
- If South does not have a 5-card suit, he doesn't want to guess which
of two 4-card suits to bid. Instead he can bid 2NT to say 'I am not
passing the double for penalties, but I don't know where our fit is,
please bid your lowest 4-card suit'
On this deal it's only 'pick a minor' because South doesn't have 4
hearts.
On the sample hand, if North's minors had been the other way round,
they could have played in 3D.
If South's hand had been, say,
Qxx
AQxx
K10xx
Jxx
then after the double, South bids 2NT, North bids 3C (lowest 4-card
suit), South bids 3D (lowest 4-card suit) and North can bid 3H,
knowing of a 4-4 fit.
If South only had one suit, he usually just has to bid it, so 2NT
implies (at least) two possible suits to play in - that's how North
knows on my sample auction that there is a heart fit.
The exception is on exactly a 3=3=3=4 distribution: South can bid 2NT
knowing that he will play in a 4-4 club fit if there is one, but if
North had, say, a 2=3=5=3 shape they can play in the 5-3 diamond fit.
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> Dave D