Miriam, I have subscribed to the notion that the withers tip outward on the bend, and the undercarriage of the horse tips in, causing a lift on the inside of the ribcage, under the saddle (lifting the rider). So when I ask Tam to bend, I lift my inside seat to allow the bend. If necessary, I stretch my outside leg down and under the belly. None of this is done to excess of course...it is an AIDING and not a MAKING.
It is shown quite nicely in a Bent Branderup video on bending. At the beginning of this trailer video http://www.youtube.com/watch?v=WrOGXN_QN6U
you can see that when he asks the horse to bend in hand (seen from above) the horse leans out slightly and ever so slightly the inside ribcage lifts up. So this is what I try not to block while under saddle (by not pushing my seat to the inside of the bend or leaning to the inside which would cause undue weight on the inside).
I have learned to manipulate Tams haunches (from the ground) to induce a lifting of the back, or by asking him to curl the haunches to one side, and I can clearly see him lift the inside ribcage.
I had been taught to push the ribcage away on the inside with my inside leg but at the same time knowledgeable people (who were telling me to do this, or to bend Tam with the inside rein pulling IN or worse...both) were telling me that Tam was NOT bending (so basically, their only solution was for me to try harder). Well no wonder he wasn't bending.
Basically, the image I had in my mind (that was placed there by people who should know better) was that his outside ribcage should bow OUT and he would become somewhat concave on the inside. Uh Uh. No, no, no. Not according to Tam. Now when he bends, my inside leg shortens because he's pushing it and my outside leg gets longer because his outer rib cage is dropping down and away.
As soon as I started to ALLOW the bend rather than pushing on him, he bent willingly and well and the bended exercises began to flow freely.
I was recently at a clinic with JP Giacomini (it was in July) and he rode Tam because he's heard so much about him that he wanted to feel what he was like. When he got off, he pronounced that Tam would not bend and that I would probably need spurs to get him to bend. I discussed it with (with a very frightened look on my face because I've never used spurs) and told him I had to try without. After JP explained how the bend should occur and how to aid it, I rode Tam for awhile. JP said nothing but I assumed that I was doing ok or he would tell me otherwise. After a very long time, JP only said (with a smile on his face) "you are getting the bend". And it's been no issue since then. Tam is SO much happier. I also got my riding position adjusted and found out that Tam wasn't lifting his back...now he is. Very much so.