It's not necessarily role play of assault, though it can be that. It can also mean that blanket consent has been given ahead of time, and so the subject is unaware of what may happen to them or when, and by the nature of it can't give specific consent for any particular action. It can have to do with pushing/exploring boundaries, a power dynamic kink, or just enjoying the element of surprise, among others.

If you're doing it right, consent can be withdrawn at any time via a pre-established non-verbal signal and/or safeword, and any bondage equipment will have safety releases.

e: In case it's unclear, I just mean that the definition is more broad than that. I don't think there's anything wrong with role-playing assault.