i very often see this IOI after a girl has acknowledge me but sometimes i wonder whether this IOI really means anything is it an indicator of physical interest or merely an acknowledgement when you guys the the hair flick what does it mean to u?? thanks

Don't read into it too much.

I think it's best to proceed by assuming attraction, it's +EV as a poker playing friend would say.

That said, I find that if there is one IOI there will be multiple. Is she picking up her end of the conversation or is it an interrogation? Is she facing her shoulders towards you and holding eye contact, or is she looking over her shoulder scanning the room for an escape route? Does she touch you? Does she react negatively when touched? Essentially, if there is a lack of overlapping signals of interest I would read that she isn't attracted but you can always test to find out.

Try and bounce her to a different spot in the venue, or to a different venue entirely. Ask her to give you her hands or to close her eyes, basically if she complies when you take the lead she is into you.