We never know what's going on underground. As long as you're getting the result and the dog isn't giving up then thats all you can ask for.
I've seen a bitch, that was ruthless on a fox, bolt them from single hole earths on numerous occasions, some would say 'oh the dog's shite, it lets them past', but I reckon a more truthful account is that she went in all guns blazing and the fox managed to get by while they were rolling about and was away.