I didn't know what to call these.  It's not "move."  The API call in question will speed up if you're already going forward, or slow down if you're going backwards, or actually switch from backwards to forwards at the minimum typical speed.  It's hard to describe, but the behavior is intuitive once you're looking at the shuttle ring instead of the underlying code.  It's basic shuttle ring behavior, but I don't know of a term for it.  Maybe "clockwise" and "counterclockwise?"