A Breath of Fresh Nothing is an in-game achievement in Run 3.
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.