I think I have a very vague idea of what this means, but can someone clarify it for me, and let me know when you feel you’re doing it right (pic or vid would help)? I just sort of tilt my upper body back…thinking the angle is the one between legs and upper body. Or is it more an outward angle…like rolling the thigh away from the horse? 
to me its the angle that you are tilting your pelvis, which when you’re riding I see visually as the crease where your thigh and torso meet.
So when someone tells me to close my hip angle, I know I need to roll my pelvis back (so that I’m more on my crotch, for lack of a better example). If I needed to open my hip angle, I would roll my forward more so that I’m on my seatbones.
You can play with this as your sitting in a chair-just roll your pelvis forward and backwards (so send your seatbones into the seat, and then away from the seat). Its a good exercise to get your hips to move independently of your upper body and legs.
What KateKat said is pretty much it.
The problem with terms like this one used by some trainers, is that it’s too ambiguous to have one particular meaning for all students.
A good trainer will show you exactly what they are asking you to do in detail first. Then if they want to attach a “mantra” phrase to it, the student will know what the trainer is asking.
If you don’t understand what your trainer is asking, tell them you don’t understand. If they can’t explain it to you in terms you can understand, or won’t take the extra time to make sure you understand, you might want to consider a trainer with better communication skills.
A true trainer is also a good teacher, not just a good rider who happens to give lessons.
Thanks KateKat, I see it clearly now. Gonna try it on my office chair aka balance ball. Been working on developing a more independent seat esp during a sitting canter so i see why this came up.
Totally agree alterhorse - I usually do question my trainer when I’m stuck (she is great) but we were in a busy group lesson all at the canter, and I didn’t reflect much on it til long after the moment had passed 
I just winged it.
Hip angle
You should only open or close your “hip” Where your upper thigh meets your pelvis. To close your hip you need only to push your butt a bit behind you. To open your hip simply put your seat bones back in the tack with a tall upper body.
A great way to figure this out is take a thin dowel, 4’ long, a quiet horse and a trusted holder. Hold the dowell behind your back with the crook of your elbows. Then close your hip angle behind you. To open, sit back on your seat bones.
The dowell will isolate your hip angle and it will be easier to understand. You can even do this on the ground before you mount.
Use a horse not rattled by a stick or dressage whip. Keep the reins, in a knot on his neck where you can grab them quickly. Any sign of trouble let go of the dowell and grab the reins.
I have horses and students I can lunge at all gaits this way. It helps teach the rider to balance over their leg, so if their horse simply vanished they would land on their feet and not fall fwd.
