I'm sure there have been occasions where you have observed a smile and you can sense it's not genuine. The most obvious way of identifying a genuine smile from an insincere one is that a fake smile primarily only affects the lower half of the face, predominantly with the mouth alone, and - the eyes don't really get involved. (from a book "Lie Catcher" by David Craig)(link1
What's the exact meaning of 'with' in the bolded "with the mouth alone"?
When referring to Merriam-Webster learner's dictionary(link2),
1)definition 4 : using (something specified)
or 2) definition 2 — used to say that two or more people or things are doing something together or are involved in something
or 3) else??
If the definition2 is right, I think it implies that 'a fake smile affects with the mouth alone' and this sentence is not correct because 'with the mouth alone' cannot be an object of the verb 'affect'. I'm so confused.
If I change "you can sense it's not genuine" into "you could sense it was not genuine", is this right? Somebody says that it is possible.
Thanks in advance.