For a long time this actually confused me, until I heard Philippe Karl expalin it. So now, I'm going to try and explain it in even simpler terms.
A little anatomy is required for this, but I'll keep it brief!
The horse's body is not attached to the shoulder by bone - we all know the horse doesn't actually have a collarbone like we do. The body and the foreleg are held together, relatively loosely (comparative to a boney joint), by the muscles of the Thoracic sling.
Think of the shoulder as a fixed point, with the body suspended between the shoulders; in the same way that a child's swing hangs suspended with the frame either side. The swing can go backwards and forwards, up and down, independently of the frame that it's attached too. Or suspension in a car; the wheels can bounce up and down over bumps, moving up and closer to, or down and further from the bottom of the car whilst the car stays level and a relatively fixed point.
The poll isn't the only thing to think of when training your horse to be "off the forehand" but you should make keeping the poll at the highest point one of your priorities.
I am a dressage trainer and general equestrian coach in Surrey, Sussex, and Berkshire. I teach dressage lessons, and hold a range of riding and equestrian clinics around the UK, and use my blog to share horse training tips, advice, and resources.