Why is Yaw 2nd effect of Roll? (and explain Trim)

[blobber]

Why is Yaw 2nd effect of Roll?

and

explain how the trim works accurately and simply - why when we change airspeed does the plane need re-trimming and therefore why trim cannot be finalized until airspeed stabilizes?

I would like to know the most accurate and correct and understandable way of explaining these to a student.

Basically the most technically correct detailed explanation remastered into the most effective description

Thanks