I've learned about this from The Game by Neil Strauss, so big creds to him.

Basically this idea is that women judge you within three seconds as someone they want to talk to or a creep, and if you say anything after three seconds that sorta solidifies creep status.

This also appears again when talking about kiss-closes, and that if you and a girl you've been talking to keep eye contact for more than three seconds, it's a good sign she wants to kiss.

Keeping this tactic in mind in the field, many girls at my college respond to an immediate hi rather than I wait and then say hi. Now the conditions can be different, depending on if it's at a social event or at a public place.

If you're walking and spot a girl in the distance walking toward you, it can be extremely hard to get within the three seconds of exposure a hi from 10 yards away. However, if you let her walk behind you, and then she catches up, they respond more warmly.