The spring growth is the light yellow to white part of a ring. I think its called the spring growth because it is the first material that is layed down when the tree comes out of dormancy and begins making a new layer of wood. It is full of pores (maybe up to a third of the thickness) that carry water and nutrients up the tree, and being full of pores it is naturally going to be less dense and therefore less strong. When you train the wood to bend, the pores are the first thing to collapse because they are essentially hollow. It is my theory that once they have been compressed down to a similar density as the surrounding summer wood, then they behave similarly. You can kind of see these pores when you chase a ring, when you cut into this spring growth it looks like little honeycombs. It also has a crunchier sound when you are in that part of the wood, as opposed to more of a ringing sound when you are in summer wood.
The summer growth is the darker yellow part of a ring that is layed down later in the growing season. My sense is that it is there more for structural purposes, to bear the weight of the tree. As such it is more dense and less subject to being mashed thinner when you are training the bow to bend. It also probably contains more of the extractives, those oils and other complex organic molecules that add strength to the cellulose and lignin that make up the structural component of cell walls. Thgis is the stuff you can melt when you heat treat a bow, and as the wood cools this material will re-solidify on the cell walls and help to hold the wood in the desired position (if you are clamping it to a form, for instance.)
If you have two 1/2"-thick pieces of osage, one that is 75% summer growth and one that is 50% summer growth, then that last piece is going to have less wood in it. It will take a little more set and also produce a little less poundage. You can compensate for there being less wood by making it a little thicker/wider, but you can't really do anything to offset the amount of set its going to take. I suppose you can design a bow that is not as stressed (longer and wider) but there is always going to be a tendency for the spring wood pores to collapse down.
As always, I supply more than you really wanted to know.