Rub "a difficulty, obstacle or objection"
In quitting, the rub is whatever allows you to give up your quit.
See, the rub is actually just an excuse.
Nothing that I know of requires smoking.
Have you considered the rub?
Is it losing a pet or loved one?
Is it a car wreck?
Losing your job or a friendship you value?
A hurricane? Tornado? Fire? Flood?
What's it going to take for you to give up on your quit and smoke?
If you think about these things in advance, perhaps it is a way to prepare your response if they happen.
The only one on my list was if I suddenly lost my wife and daughter in an accident.
Thankfully, that never happened.