If partner opens 2NT [20-22], responding 3♠ is showing both minors, at least 4-4 and in practice probably at least 5-4. With no interest in playing in either minor, opener rebids 3NT, otherwise he rebids 4 of the minor he's interested in [i.e. holding at least 4]. Responder treats that as a RKCB asking bid, one bid up showing 1/4, 2 bids up showing 0/3, etc.
If opener rebids 3NT and responder now bids 4 of a minor, this is similarly a RKCB asking bid.