From the Cavalry Manual: what is Throughness?

“As the straightness improves, the throughness of the horse will improve as well. The pushing capacity of the hindquarters can now go its way through the horse up to the mouth and makes the horse give to the pressure of the bit, bend the neck and chew on the bit.”

“This is the natural way the horse achieves the correct ‘going through the poll’. It would not be correct to achieve this ‘going through the poll’ by brutally pulling neck and head of the horse.

Instead it must be the result of the hind legs pushing towards the quiet still hand. This is the only way to fix the neck to the wither. And only if the neck is fixed to the wither it is possible to link forehand and hindquarter.”