You say to a new chick "I could never be your boyfriend"

Occasionaly they will say something smart ass like "you are so right"

but often they say "why not?" or why do you say that?

her cards have been read at that point. It's not a bad way to see if she has any interest at all.

If she ignores and walks away, could mean a million things, but if she says why not, that's a good sign.

Yup. This is pretty standard disqualification.

yet different than disqualification. You're just seeing if the thought of you being her boyfriend is at all pleasing to her.

No, it's disqualification. You say you guys wouldn't work out, she asks why because she perceives it as a challenge, and you give a BS reason that doesn't make much sense. She sees it as more of a challenge, starts thinking about the possibility of you two in a relationship of some kind, and it builds more attraction. It's pretty standard stuff, just about every method of seduction uses this kind of technique.

Just because a girl asks why doesn't mean she wants you to be her boyfriend, it's curiosity. I've had girls I wasn't interested in at all say we would never work out, and even though I'm not interested, I'm still curious as to why she thinks we wouldn't work.

No, you added to it. I know it doesnt mean she likes you if she asks why, but if she cares enough to ask why, she at least respects you.

Dont look too much into it, It's just a fast indicator.

Well of course, you aren't just gunna say "We wouldn't work together" and then stop the conversation once she asks why. Maybe that sentence in and of itself isn't a disqualifier, but the continuation of the conversation leads into disqualification and attraction building.

It's a great technique, I'm not arguing that with you.