If you prune in August the tree will still grow slightly but by that time you should not fertilize anymore which helps. This allows the tree to harden up before the winter.
Summer pruning is meant to control size/height and to promote new flower bud growth. So far I have only cut laterals or what some call side shoots from the main structure or arms. My trees are very short and so far only need to cut the laterals to promote flower buds for fruiting. I will end up cutting the main structure arms soon since the are now around 6 feet tall.
I believe but not sure that the 1/3 rule applies in spring when they are dormant. This is the technique I use for all my fruit trees. The apples have only been in the ground for 3 seasons and I did not need to take off 1/3. Most of the pruning was done early on to get the structure and shape bases on how the tree was growing.
Keep in mind some apple trees have spurs which are pruned differently. I have no experience with that type of apple tree.
Had 6 apple trees way back but removed them because of size. Bought the standard size at the local brick and mortar store. I knew nothing about fruit trees and struggled because the varierty wasn't best for my environment. I started over with the newer highly disease resistant varities. I got fruit with the brick and mortar trees but considering I had 6 trees at that time it was a total failure. Best learning experience in fruit tree growing I have had. Made so many mistakes but thats how I learned to prune fruit trees.
Here is a link to what I consider to be the best apple tree pruning video. This guys is great but not to many followers or views. Definitely doesnt get enough attention or credit for his experience and knowledge. Does a great job explaining and pauses the video for clarity.
This one is my favorite. This is part 2 search for part 1 to start.
MOD