This is obtained in Chapter 5. When you are looking for soap, one of the puzzles in the left area of this chapter requires you to time the jumps of three yamyams onto the top of a paper airplane to push out a bar of soap.
After pushing out the soap, the yamyams continue flying from the left area to the right area. To unlock the achievement, switch screens from the left to right with
and watch the yamyams fly away on the paper plane.