Opening to middle game transition
- a player plays in the opponent's sphere of influence, and a fight starts
- a complicated standard sequence (joseki) extends into a fight
- a player makes a mistake in the opening, and his opponent punishes him by chasing him into a fight.
In all cases, groups of both players run out into the center. This is the distinguishing feature of the middle game.
Another way of looking at it is that the opening is over when there are no more big points to be played. In actual games there are often early fights which reach a point of equilibrium and then the game switches to taking big points again. In a sense, then, there has been a bit of middle game activity and then a reversion to the opening. Of course, this moves the burden of defining the opening to defining big point.
Actually, there may not be a clear border between the opening and the middle game. For example, probe moves against a corner formation may be played in the opening and these sometimes lead to a local fight. You could also say that joseki sequences are fights locally.