With unconscious nail-biting the habit is in control. You are not because the habit is facilitated by unconscious involuntary hand movements. We have these instinctive 'hand-to-mouth' inclinations from childhood and we consciously control them but when the mind is focused elsewhere habits like nail-biting can develop.
Nail-biters know it is the first bite that must be prevented as that is when most damage is done. A bitten nail cannot be unbitten so then it is a matter of conscious damage limitation.
The Tango Program is about prevention. Tango wrist and waistbands will alert your conscious mind/willpower to pre-empt those damaging episodes, to stop them before they start.
Furthermore by following the Tango Program you are forming a new habit which prevents other habits from developing or breaks them if already formed, not only nail-biting but also finger/thumb sucking, hairpulling, skin picking or any unnecessary face touching.