In general, any low-level double after opponents have bid a suit (especially if they have found a fit) and partner has passed is generally played for takeout: Balancing (commonly called protective in England) doubles occur when the doubler had to choose between passing and ending the auction or bidding again to reopen the auction.
A common expression is "borrow a King" meaning that the balancing doubler can add 3 points to their hand and then bid according to strength guidance.
[7] South, holding ♠ Q863 ♥ 84 ♦ A85 ♣ K1054 can now double, expecting to find partner with strength and likely length in hearts, allowing him to pass for penalties or make a bid and this is a case for bidding with a little as 6HCP[7] with the right holding, shortness in hearts is particularly important and 4+ in all other suits would be ideal.
Although the bidding is a level higher East-West have found a comfortable fit and will probably be happy to play there.
Competing at the three level might be risky as the major auction says very little about the minor suit holdings of East-West.
As above South should be keen to compete (assuming East - West play traditional methods rather than inverted minors) but the auction has suggested that east–west don't hold length in any other suits (and the majors in particular).
However, he can expect the partner to have shortness in hearts and likely 4 cards in spades or diamonds, and thus make a balancing takeout double.
In order to protect against such situations, opener is obliged to double even with thin values, whenever his distribution is suitable.