Can you get a horse to open its mouth by pinching its nostrils?

I've seen this in books too.

No, you can't. Horses do not have a cross-connection between the windpipe and the esophagus like we do. They cannot breathe (or vocalize) through their mouths. You make them open their mouth by sliding your thumb into the bare gum between their molars and incisors and pressing gently.