I think we've had this discussion before and most of us agreed (then) that we didn't approve of this technique to "prove" a submissive is willing to safeword her way out of trouble.

I prefer, in the heat of the moment, to ask my submissive if she remembers her safeword and have her tell me.

After a few reminders I feel assured she hasn't forgotten it... and if she should happen to struggle and asks me to stop or says "no" I feel perfectly fine reminding her that 'No' is not her safeword. Once, reminded, if she doesn't use it, then I am happy knowing we haven't reached her limit.

The technique of purposely pushing a known limit as an 'object lesson' borders on abusive. IMO. The more seasoned the dom, the less likely he'll use this tecnique. Also IMO.