A few years back i was playing 5's in a tournament in inverness. One of the teams managed to get the then ict manager john robertson down to play for them. Little cunt to play against, really strong on the ball and was pinging them in from all over.
Some players are just more able to get into a game quickly than others when coming off the bench. To me this would be a combination of instinct and, as you say, having enough upstairs when watching from the sidelines to figure out how to go about the business when taken on.