Having a stable lower leg is vital to having a stable position.
It can take a while to train the muscles to hold this position and have the strength to do so as well.
I'll give you an analogy as to how the lower leg stabilises us.
Think of a horse shelter with four posts and a roof. Our legs are the posts and the roof is our upper body.
If we just sit the posts on the ground and attach the roof so it is all balanced it will be fine until an outside force causes it to become unbalanced, such as wind or a horse scratching on it. Once this outside force makes it unbalanced the shelter is going to fall over.
This is the same with our position. If your lower leg is not stable you will be fine and feel like you are balanced, however if you experience an outside force that makes you become unbalanced you are going to fall off quite easily. Something as simple as the horse slowing down before you are ready is enough to make you become very unbalanced.
Now back to our shelter. If we secure the posts into the ground then this gives the entire shelter stability even against outside forces. The shelter is going to remain strong and balanced against the wind or the horse scratching on it.
Now if our legs are 'locked' into position and are able to keep us stable it will take a lot more force to make you become unbalanced.
Keep up the great work everyone and Happy Riding!!!!