I couldn't even find a description of this that made sense and it doesn't describe anything like this in the How-to-Play section of the game, but the best description I can give is when a droplitz is collected by a bottom collector after the path it came from disappears. Previously, it would have just went into a wall. I'll try for an example: Say you had a path split towards the top and you were able to create paths for both almost all the way down. Now, finish one of the paths but leave the other open even though all you'd have to do is turn a dial or two. Let that first path finish and disappear. All those dials will disappear and you'll have spare droplitz still heading down the incomplete path. Finish that second path and the droplitz collected will count towards this achievement. I got it without even knowing what it was in my first game, so you really don't need to worry about its complicated nature. Just play and it'll happen pretty quickly compared to almost all of the other achievements.