Danny turns the right knob twice, looking upwards towards the number afterwards as if expecting a response - which he is given in the form of a flash of the twins. In the Games Room Danny threw darts against the wall of the hotel and it responded with an appearance of the twins, and with his actions / expectations here he shows he's learned from that experience and is conscious of the forces he is 'playing' with. At the end of the movie Danny will kill his father, trick him and trap him in the hotel. Danny is the murderer in this movie, and this exploration of room 237 after Hallorann's explicit warning shows us the extent to which this murder is premeditated.