Level 19 is a level that the conveyor belts are powered on making it start when the level loads. There are four conveyor belts all pointing to the right with 10 penguins on the belts ready to move. They will be in the following order: I,I,I,G,I,I,G,G,I,I (I = Imposter & G = Genuine). You earn the achievement by only using 4 of the 5 "change direction" actions in the level.
When the level begins, place you cursor over the first conveyor belt *don't click*. As soon as the fourth penguin (genuine) crosses to the second belt, click the first belt. This will cause the first three imposters to hit the floor to the left. Next move your cursor to the splitter and get ready to watch the direction arrow to send genuine ones to the box and imposters to the floor to the right. Keep in mind the splitter changes direction each time a penguin falls into it and you only need to change the arrow when it is pointing the wrong way.
Splitter commands: first penguin drops right, CLICK
second drops right, third drops left, CLICK
fourth drops left, fifth drops right, CLICK
sixth drops right, and final penguin drops left. In the end, the box will have 3 genuine penguins, the floor will have 7 imposters, and you will have one "change direction" action left which will trigger the achievement to pop.
I made a video for the sequence: