WR round these parts indicate to the official by either pointing forward (I'm on), or pointing to the rear (I'm off)...and the wing will confirm their intent or advise they need to adjust
Trying to de-code what they're saying verbally is a challenge with mouthpiece, crowd noise, etc.
In as far as 'legally, by the book' being a back (i.e. breaking the center's waist or not), as along as there's some sort of stagger (i.e. the 'blade of grass' axiom), no one gets too technical over that round these parts.