A couple of Saturdays ago we (the usual suspects from the Saturday amateur group that I play in at the Nihon Kiin) were out after a hard day's work with Kawai Shoji 5p (btw, the 3rd generation pro mentioned by TMark in this post). He plays frequently online. He prefers 20-second byo yomi. I asked him what he thinks about during those twenty seconds. He said that he uses those online games only to exercise his pattern recognition. He immediately chooses candidate moves based on his memorized patterns and then has to validate their applicability to the current situation within the time limits.
He also said that he can tell amateurs from pros by the moves that they choose. Even very strong amateurs choose different moves than pros. It is not that the pros always choose better moves. Rather they have accumulated similar types of moves in their available "inventory" of choices through their professional studies/playing careers.
Kawai sensei, another fan of Japanese sake...