How is "having a witness present" during a paddling protecting the child ?

Don’t teachers and principals protect each other anyway? If one goes too far, I’m sure the "witness" isn’t going to tell on the paddler, is he?

No, adults won’t protect each other. A witness does protect the student, just like having a person in a classroom observering the teacher protect the student… it makes the paddler more conscious of what exactly they’re doing.