When Haku said to Chihiro don't look back when they said goodbye. What would've happened if Chihiro did look back?

My guess is that it was part of the spell that kept the spirit world and the real world apart. Remember that when she tried to go back on her own the first night, she ran into a lake instead of the path she had walked with her parents. Therefor I think she would be unable to return if she'd looked back. (Not that I have any idea how this works anyways)

I think if you take the whole movie in a coming-of-age context (which I think it really is. I think I read it somewhere), what Haku means por not looking back is por not dwelling on the past and keep on moving progressivo, para a frente in order to mature and be stronger. Through the whole narrative of the film we saw Chihiro grow as she encounters each scenario. I think. This is just my interpretation.

Well, I don't really know...
I guess nothing would happen. Haku just didn't want Chihiro to feel even worse for leaving...I know she wanted to leave anyway, but it's kinda hard to make a new friend and then leave them behind like they never existed...

I'd have to agree with the above user. I always thought he meant that if she ever looked back, then she won't be able to return to her início world apart from the spirit world. I guess she might stay stuck in the spirit world forever.

She probably would have been sucked back in/unable to leave the spirit world. Like the legend of Orpheus saving his wife from Hades' realm, both being warned not to look back, but he accidentally looked back before leaving the cave and his wife got sucked back in and was never able to leave the Land of the Dead again.

In fact, if memory serves, many mythologies have similar stories, warning mortals not to look back before they leave some mystic realm (the realm of the dead, the spirit world, fairy world, whatever) lest they be unable to leave.

That might be what Miyazaki was referring to there.

IMHO, the Spirit and the Real World could represent a time continuum, where the Spirit world is the past, and the Real world as the present/future.

I also think of the Spirit world as a sort of cursed wishing well, where whatever you wished for could be granted, but if you fall prey to your desires and forgets who you are, you'll be stuck under Yubaba's control, forever. It's like how Chihiro's parents turn into pigs, and Haku not being able to leave because of his desire for power. Hence, we could say there're two conditions for leaving the Spirit World: 1) remembering who you are; 2) overcoming your desires.

Chihiro was probably able to go to the Spirit World because of her wish to go back to the past (to not change school), and the Spirit World answered her wish. In her moment of weakness, she forgets who she is (represented por the event in which Yubaba took her name).

But Haku was there to help Chihiro remember who she is, effectively foiling part of the curse. As we mover along the story, Chihiro become stronger despite temptations (No-face offering her gold) and obstacles (Yubaba's attempts to keep her in the Spirit World, staffs at the bathhouse bullying her because she was a human).

So, if Chihiro looked back to the Spirit World, it could mean that she was unable to overcome her desires, which also mean that she couldn't break the curse. Conclusion is, she'd probably get stuck.

Chihiro would have probably been cursed to become a dragon, but oi at least she doesn't need to leave Haku (based on a fanfic where Chihiro became a dragon)