This achievement is required to complete some other achievements, such as Figure Skater.
The achievement relies on the fact that gravity doesn't apply outside a tunnel. This allows you to float past some of the level. However, there's a weak force pushing away from the center of the tunnel, which will make it harder and harder to return the longer you wait.
While outside, you can only move left and right. Therefore, you want to leave through the side of the tunnel, because if you leave through the bottom, moving left or right can't get you back inside. Ideally, you want to leave the tunnel just before the peak of your jump, so that you're moving upwards very slowly. The best tunnel shape for this is a square, which is why the achievement description recommends Level 19. There are also a few other levels that are easy for this achievement. For example, it is quite easy to do it on level 1, exiting through a square in the wall, then entering back into the tunnel in the lowest hole.
It's recommended to not use the Pastafarian or Child when attempting to do this achievement, as the Child's low maneuverability and Pastafarian's light bridge can prevent you from getting back in or out.