If you’ve gone to light, active aids with your legs (squeeze-release, squeeze-release) and you’re still not getting your 35-degree angle, you can transfer part of the responsibility for going sideways out of your right leg and into your right rein to help give the horse the idea and make him more athletic.
If you’re supporting enough with your left rein (to keep the neck straight), your horse will ricochet off the right rein, and his hind quarters will swing in quite dramatically to the left. Eventually you wean yourself off the opening rein.
It does three things: it makes sure you don’t end up leaning in the wrong direction; it gives the horse the idea of how much he needs to bring his hindquarters in; and it prevents you from falling into the trap of using too strong of a leg aid. And your horse mentally begins to understand, and physically he develops the suppleness to cross high up on his knees and hocks. If he never uses his body in that way, he’s not going to develop the suppleness to use his body that way, so this gives him the chance to actually do it and become more athletic.
By teaching your horse the turn on the forehand and leg-yield, he will increase in suppleness and become more athletic. It’s a great warm-up exercise for horses of any level, and will increase the quality of all of his work on the flat.
Further Reading
Leg Yield and Turn on the Forehand