The knee on neck/throat choke is a controversial choke in BJJ as you are basically putting your knee on your opponent’s neck which can seem quite brutal.
Nevertheless, it is a very effective submission.
This is an instructional video on the knee on neck choke to armbar performed by world champion, Jena Bishop.
Gracie Humaita instructor Jena Bishop provides some insight on making the knee on neck choke work along with a transition to armbar:
Here is another interesting option:
Here is Cyborg doing the knee on neck in competition: