What is the mechanism behind why effortful practice out-teaches easy practice?

Storage strength vs retrieval strength — successful practice done when retrieval is HARD (low retrieval strength) yields the biggest durability gains; fluent easy practice teaches almost nothing.

Bjork's framework gives every memory two independent values: how deeply it's wired in (storage strength) and how accessible it is right now (retrieval strength). The key insight is that the act of retrieving is what builds storage strength — and the more effortful that retrieval, the bigger the boost. When practice is easy because retrieval strength is already high (you just saw the answer), pulling it up costs little effort and banks little durability. When you have to reach for it, the successful reach is what wires it in. Desirable difficulties are all ways of deliberately lowering retrieval strength so that each success does more work.