Not obviously means you should do the opposite

absolutely fascinating! can you elaborate on this?! I can’t put a context to this, like in what context does “not” means to do the opposite?!

It is a negation - so anytime you combine it with a verb (grammatically).

Ex:

I have seen the movie --> I have not seen the movie

When combined with the verb "do" (and giving a command or instruction) it would negate the verb "do"

Ex:

Please do run on the lawn --> Please do not run on the lawn