Knife In The Wall (Daf 16)

This is the second words of a question. The shochet will take the beheima go under the knife then he's going to try to shake like this. This way when the knife is on top that is the bottom there is a kosher shifta. Now when it's the opposite then shake the simanim like this it's pusul it's oser because of you cannot control yourself not to be pushing on the knife.