There are a lot of complex answers to this, but I am going to try for a simple answer based on the efficiency priorities from the different stages of the game.
So, when you are playing in the Opening, the priorities are
- approach unclaimed corners
- approach an opponent’s corner or enclose your own corner
- create or prevent large extensions
- create or prevent small extensions
- once all the corner/side territory is claimed - make vertical moves into the middle
However, the transition from Opening to Early Midgame is not always smooth or easy to see, and a lot of players start taking advantage of Midgame efficiency priorities before the Opening is completely over.
In Early Midgame, one of the simple priorities are to push into the middle from both sides of a large extension to expand your side potential to create a big moyo - and conversely, to make moyo-denying moves (shoulder hit, big jumps, etc) to prevent your opponent from making a moyo.
The way I think of it is
- in the Opening one is focusing on 1 dimension (length) - “how much of this edge can I grab so I can make a large extension”
- in the Midgame, one needs to start thinking in 2 dimensions (length x width = area) - “how can I push inward from both sides of my large extension to make a moyo”
Now, of course it’s not always that simple. With higher level players, the first 50-100 moves might be a life-or-death fight for one corner that slowly spreads across the board before they even “finish” the Opening. Other players will make moves that result in their opponent making weak groups that need to run to reinforcements, and then they use that direction of play to create large extensions or expand moyos, etc. So yes, there is a lot more to the story than just following the efficiency priorities, but I feel it’s a good tool to use to put those other strategies in context.
This article I wrote might be re-hashing stuff you already know, but hopefully there will be some useful nuggets of info in there: