Using your seat to collect the rear all the way through the spine; now how do you bring the head down? A green or retrained horse that isn't used to moving with their rear ends under them. You can drive them, make them pick up and reach under themselves better. They're still learning.
Because you don't throw your reins up their necks and leave them to fend for themselves
A young dressage horse, fresh from the breakers so has no idea about collection, going into a 'frame' etc. is ridden very forward, motoring off their hind legs, into a very light, elastic contact on the bridle. The contact is essential for connecting the hind legs, to the back, to the bridle and to the riders hand.
It does not take long at all (in fact, most breakers under a good, unobstrusive rider, will be starting to stretch from the wither into the bridle within only a few rides after backing.
All we care about, is that the horse goes forward off the leg with active hind legs. He is ridden in this manner until he develops the srength to develop a little more carrying power behind, and can then begin to slightly shorten the frame. This is when the horse will start developing a good reaction to the rider's seat. The seat takes the job of the hand, in becoming the 'restraining aid' and can begin to influence the speed and carrying capactity of the hind legs.
In creating drive in the hind legs, with the 'driving aids' (riders leg), and containing this power through the seat to control the speed and carrying capactity of the hind leg, and completing the circuit with the hind, keeping a light contact with the horse's mouth - he will naturally begin to come into the typical 'frame' that you see in a dressage horse. The stronger he gets, the shorter and more uphill this 'frame' becomes. No need to pull on their mouth to pull the head down. The contact and restraining aid of the seat does the work, and saves the horse's mouth.