Disregard the motivation problems, that's just bad phrasing.

I'm wondering how well it would work for the senior programmer to explain a very small, 5-minute task to the junior, then work on something else while the junior guy finishes the task, then check the work, and repeat.

I'm more interested to hear if anyone has ever actually tried this, and if there were any surprises or if it didn't work, why didn't it work?