This is a principle taught in the Alexander Technique of movement that applies to so much learning.
Sometimes as teachers we see the final product and we forget the process that lead to that outcome.
End gaining in music looks like learning pieces that are too hard. You only learn one really hard piece a year. You become a one-trick-pony, but you win that competition with the one piece you know.
Conversely, not end gaining means you slow down. You learn all the pieces in a method book. You play lots of easy pieces while you work on that difficult concerto. My best students put in the work and don't skip steps.
I had a high school student learn the 2nd Shostakovich Piano Concerto. While he was learning it I had him go through a dozen Kabalevsky children's pieces (see problem solving). It was much easier to work on tone, rhythm, sound, and gesture in these little case studies.