Well, I've always caught that you've to sit on your butt and butt bones and you can even perhaps lean very, very slightly back on a saddle. That's how I've ridden now that 8 years and during those years I've found my position where I stay comfortable and balance. Now that new teacher tells that I lean my back behind too much and besides my butt bones I have to sit also on my pubic bone. The odd part for me is that I feel, in the position where she leads me, that I lean very strongly forward. My butt just lift out on a saddle and my butt bones lose their contact with the horse. When sitting in that position, I feel that I just sit on my pubic bone and umm... woman parts of my body. I just feel the position is a bit unstable and if I lifted my butt more I'd have a great light seat. I don't know if that's how it should be.
None of the teachers told me about this at my previous stable. On the other hand, I know my current teacher is a qualified teacher, perhaps more qualified that teachers at the previous stable where they didn't notice your position that much. Also galloping (the gait that I've never preferred that much and which feels a bit unpleasant for me) is better. With my 'ordinary' position I occasionally feels that there is a some kind of brake on and the horse doesn't want to move ahead that freely. The problem seems to be better with the new position offered to me but I still don't know..
I know you've never seen me ride and this will be the thing that I'll finally solve out with my teacher but since I'll see that teacher not earlier than after a month and it's never bad to discuss about issues in theory with other riders I'd ask a question: Where do you focus your weight when you ride?