First East's opening bid. With a 6-card suit. if the hand is too strong to open a weak two (and it is with 10 points) open at the 1 level. There is no gap between a 1-level opening and a 2-level opening.
Now we come to West's response with 21 points we can certainly respond with a new suit at the 2-level but a 2♥ response would show a 5-card suit. So we must "make up" a bid. Our choice would be 2♣ .
Okay, now for East's rebid of 2♠. We know the spades are poor with Ace, Jack at the most and with only a poor 5-card suit East would either be opening 1NT or rebidding notrump. It follows then that this shows 6+ cards.
Now West must think, "Is the 5 level safe? Can your partner be Aceless? If they had no Aces they would hold a maximum of the ♠J, ♥K,♥J, ♦Q, ♦ J, ♣Q, ♣J. 11 points but the 5 level is safe and so 4NT is okay and we don't have to pay Mr. Blackwood 1$.
After 1 key card what is going to be the safest contract if you can really trust your partner? Click "answer" to find out.