Are you in a dressage saddle or doing dressage tests in a jump saddle? The way they position your leg is totally different. I find posting in an English saddle, jump style, to be very difficult.
I don't really understand whay you mean by "locking your hip". But I like to think of posting in dressage as being 50% from the thigh and 50% from the stirrup. Since you are in a vertically aligned position, it should be akin to how it would feel if you were standing on YOUR feet, kept a good vertical alignment of your spine and then rythmically bobbed up and down by bendin your knees and straightening them, always in a position that you can stay balanced on your two feet. You do move your hips forward some but you shouldn't be "slinging" your self up and out of the saddle, back and forth. You should be posting on YOUR balance, as it you were still standing.
I don't really understand whay you mean by "locking your hip". But I like to think of posting in dressage as being 50% from the thigh and 50% from the stirrup. Since you are in a vertically aligned position, it should be akin to how it would feel if you were standing on YOUR feet, kept a good vertical alignment of your spine and then rythmically bobbed up and down by bendin your knees and straightening them, always in a position that you can stay balanced on your two feet. You do move your hips forward some but you shouldn't be "slinging" your self up and out of the saddle, back and forth. You should be posting on YOUR balance, as it you were still standing.