The reason why you must always respond to partner’s One-of-a-Suit Opener with six+ points is that partner can have 19 points. 19 + 6 = 25: game (well, 3NT, 4♠ & 4♥ - 5♣/5♦ require nearer 28, 29 and should generally be avoided).
However if the opponent sitting on your right bids, you no longer have to bid with six-seven-odd points, for partner now has another bid.
Exercise: Partner, South, opens 1♥ What would you do as North if West passes?
Hand A | Hand B | Hand C |
♠ J 7 4 2 ♥ 7 4 ♦ A 9 5 2 ♣ J 5 2 |
♠ 9 7 3 ♥ K 6 ♦ J 7 4 3 2 ♣ K 5 2 |
♠ 4 3 2 ♥ 7 ♦ K J 7 4 3 2 ♣ Q 7 3 |
However say West bids 1♠, the auction going 1♥ from partner, 1♠ on your right. Does that change things?
You bet they’e different! When West bids, the pressure is taken off you, as partner now has another bid. Your 1NT bid remains as nominally 6-9, although in practice nearer nine than six, but you are actually expressing the opinion that you want to play 1NT. It is not some nebulous dustbin bid any longer.