Dallas v Philadelphia

[Bullycon]

Sometimes, it is the ball. Sometimes, it is the feet. Sometimes, it is the body.

In NFHS, it is a foul if either foot is beyond the neutral zone (i.e. the forward point of the ball when it is marked ready for play.) Thus, the play as you described it would be flagged for illegal forward pass.

In NCAA, the entire body must be beyond the neutral zone. Thus, your play is legal.

My 2006 NFL rule book states the entire body must be beyond the line of scrimmage. Once again, a legal play as you have described it. It's possible this rule has changed, but I doubt it.