a minmum 2-suiter to just short of a jump shift.
Some example hands might be:
xx, x, AJxxx, AKxxx
AKx, x, AQxxx, KJxx
or
x, AQ, AQJxxx, K10xx
Point being that in 2/1 parner could hold quite good hands and still have no better bid available than 2C, which means that with hands that can still produce game it is important for responder to keep the ball in the air, so to speak.
But then you have to realize that 2/1 was designed for imp play and is therefore geared to improved game and slam bidding and less so on accuracy in partscores.
Ergo, in my thinking the only way to keep this thing intelligently alive iis with a false preference to 2D - and as sure as this happens it will go all pass and partner will hold - x, Kxx, AQJx, Kxxxx. Ce la vie.

Help

1D-1S
2C-???
It's the last board of a tightly fought match. You are quietly kibitzing this hand - (you have a hefty side bet on this team) when the player hands you the cards and claims a pressing need for the restroom...an hour later everyone is still waiting and the director forces you into the fray as a substitue - what call do you make?