2,515. The Definition of "Displacement"

Hilchos Shechita 3:14

“Displacement” means that either the esophagus or the trachea – or both – were moved from their proper location before shechitah was finished. However, if one cut a whole sign or the greater part of it in a bird, after which the second sign was displaced, then the slaughter is valid.

Hilchos Shechita 3:15

If one of the signs was displaced first and one cut the other sign after, the shechitah is invalid. If someone cut one of the signs in a bird and subsequently found that the other was displaced and we don’t know whether it became displaced before or after shechitah, there’s a doubt as whether the bird is a neveila.