A partnership possesses the balance of power or balance of strength when it has a higher number of high-card points (usually a significantly higher number) than its opponents.
As soon as West opens the bidding, East knows that the partnership holds the balance of power. However, this becomes apparent to the other three players only after East makes the strength-showing 3NT bid.
When you hold the balance of power, you usually expect to buy the contract or to double the opponents if they outbid you.