To make a not-vulnerable overcall at the one-level, it is generally accepted that one needs 8 HCP’s and a good suit. Overcalls with eight points will always be lead directive, whereas overcalls with a full opener may not be. However, the one-level overcall guidelines do not apply when both opponents have bid.
Typically when both opponents have bid, a one-level overcall promises five tricks not vulnerable and six tricks vulnerable no matter how the suit breaks.
West, with 14 HCP’s, no five-card major and equal length in the minors, opens One Diamond. East bids his four-card heart suit and South overcalls One Spade.
South has a two-loser six-card spade suit and an outside bullet (ace). He will get five tricks no matter how the cards are distributed and may even get more.
West makes a negative double showing three of partner’s major and four clubs. If West had four hearts, he simply raises partner. Those players who use support doubles will notice the similarity.
North raises spades, and East with his quacky seven points passes. If East bids Three Diamonds, he shows ten points. West has already doubled and partner passed so he passes the auction out. Three diamonds would be a good contract but East and West cannot get there without misrepresenting their hands.
The opening diamond lead is won with the Ace, and declarer sees that he has no short-hand ruffs so he sets about drawing trump. Once the Ace is knocked out, and the rest of the trump are drawn, declarer will take the club finesse twice. Declarer will lose a card in every suit making 140.