Understanding What Bio-Mechanics of horse and rider really means

There is so much information regarding Bio-Mechanics of horse and rider that i really don't know where to start.
So i decided to start with our seat. Really that is where it truly begins.
If you don't have an independent seat you have nothing.
What is a independent light seat?
We hear all the time to sit with a light seat but no one ever explains how to do it and how does it feel.
The reason for a light seat is so we are not seating heavy on our horses backs and preventing them from lifting it. A light seat allows our horses to lift their backs and then engage their hind ends.
A light seat also means we are able to support our own weight as we sit in the saddle which allows us to now influence our horse.
When we start to learn to ride we tend to use the stirrups and reins for our balance.
The real purpose for stirrups is to frame your legs, that is ALL!!!!!!
When you put your weight in the stirrups you are pushing your body away from the horses back. This hollows out their backs and puts you further away from their backs.
This now limits how you influence your horse. The closer you stay to your horses backs, the more you can connect their hips to their shoulders. When you are far away from their backs you are now only influencing their legs.
What is an independent seat?
An independent seat means you are able to influence the horse and not  the horse influencing you.
Example:   When you can ride any gait and are able to do this without the horse moving you at ALL!!
When the horse moves you that is a sign that the horse is influencing your body and it needs to be the other way around. Remember you are the lead dancer. The horse should be looking to you for what to do, how to do it and when to do it.
The first picture is a rider with a light seat and the second one is a rider with a heavy seat. Go ahead and try it and feel the difference.