2,488. Checking the Signs

Hilchos Shechita 1:11

If one cut half of each pipe, the slaughter is invalid, even in a bird. If the trachea is half-slit and one cut a bit more on the slit, making the cut the greater part of the pipe, then the slaughter is valid. This is so regardless of whether he started on a whole trachea and reached the slit or whether he stuck his knife into the slit until it cut the greater part.

Hilchos Shechita 1:12

A shochet must check the signs after performing shechita. If he neglected to do so and the animal’s head was removed without him checking, the animal is a neveila even if the shochet is a skilled expert.