There are different varieties of riding with flexion/bend: 1 There is 'in position' with merely flexion at the axis (where the rider sees the outline of the inside eyelashes) which all horse should have at all times. 2 there are the varieties of shoulder in which are: 2 shoulder fore (a step onto a 20 m circle ridden straight ahead), 3 shoulder in on three tracks (a step onto or the first stride of a 10 m circle ridden straight ahead) and 4 shoulder in on four tracks (a step onto a 6-8 m volte, depending upon the gait).  The forelegs lightly cross, the hindlegs do not.

So, the first question is can the rider/horse produce a nice circle of 20m/15m?  Is the horse laterally flexible on the circle?  Does the circle 'originate' in the (positioning of )rider's legs?  (Inside one closed to the girth, outside one stretched down and back and passive)  Can the horse 'enlarge the circle' and move into the outside rein?  

Remember that the idea of a spiral seat (ie riders shoulders parallel to horses, hips to hips), and reactions to the legs. IF those things are in order (and perhaps the ability to do a mobile turn on the forehand) and the horse reacts to pulsations of the inside leg, then rider a 15-20 m circle, and then start/step onto the circle and almost immediately have outside hh (remember you are looking where the horse looks...onto the curve.  The outside hh prevent the horse from leaving the wall, but it cannot be hung on or the horse will change flexion.  For this exercise to be successful the horse MUST be reactive to the inside leg (close to the girth....not drawn up or back) and its pulsations.  Exit exercise after a few steps by either doing another circle or by leading the horse back to the wall.  It is a game of reaction to inside leg, limiting by outside rein.  After the first step onto this exercise there is NO need for inside rein holding per se.

Hope this helps.