I have encountered in my professional training some studies that showed that if you teach material to someone else, your retention of that material is much better than if you merely study it. (Sorry, I do not have the references handy.)
However: this can't be the only (or principal) strategy employed, because exposure to new material is obviously necessary. It would be a question of proportion. I'd say that occasionally having to present one's knowledge to others solidifies that knowledge-- but it does not facilitate new learning.
I wouldn't want my child working as a tutor daily while s/he should be learning new material, but it could be OK with me if used appropriately.
Like all tools, this one can be used or misused.
