2,512. The Definition of "Concealment"

Hilchos Shechita 3:8

Let’s say that someone slaughtered a bird and paused, but he doesn’t know whether or not the esophagus was pierced. In such a case, he should go back and re-cut just the trachea in a different spot. He should then leave the bird alone until it’s dead, after which he examines the trachea from the inside. If a drop of blood isn’t found in it, we know that it wasn’t pierced and is valid.

Hilchos Shechita 3:9

“Concealment,” for example, is when one inserts the knife in between the two signs. Whether he then cuts the upper sign above the knife or the lower sign below it, the shechitah is invalid.