Peace,
In marriage, a women must obey her husband in all marital issues in exchange to the gift and caretaking done by the husband. He has the upper hand.
Nushuz means total rebellion and disregard of the husband. Nushuz doesn't come immediately but in stages.
If you fear your wife is going to a state of nushuz, you must act, otherwise, she will disregard you completely ie. Nushuz.
To avoid nushuz, doing the three steps including beating, the final and corporeal punishment, is enough to avoid nushuz.
Peace,
Rami