When one of the partnership opens 2NT, he already gives a precise description of his hand. But even so, his partner has many bids available to ask him to refine his description
With 6 HCP, it goes without saying that West intends to impose game facing his partner’s 2NT opening. In order to most accurately describe his 5-4 in the majors, he starts with Stayman. Then, after East’s negative answer, he employs a useful convention, called Smolen. His 3♥ bid guarantees five Spades and four Hearts and allows the strong hand to play the contract. Here, East logically corrects to Spades to show his three cards and thus the best game is duly reached.
Be careful, (RKC-) Blackwood only exists if the side has explicitly established a fit. When Stayman is not successful, East, who has 12 HCP but no fit, can therefore employ 4NT here. Now this bid has a purely quantitative (propositional) meaning. As West has a minimum opening and no extra values he passes and the abominable slam is avoided.