The idea that bridle horses ride mostly off of seat and legs is not so. They ride a great deal off of the reins and bit.
The bridle horse rides with a romal, so the rider cannot 'cheat' the horse with the customary finger between the reins. The hand encircles both of the reins and slides up and down the reins very little. The reins are seldom 'split' for training -- they are ridden 99.9% with the hand wrapped around both reins. If a horse requires two handed riding to correct the nose position or to achieve a little more 'bend', a snaffle or 4 reins are used.
Bridle horses, as a result of this lack of 'bend', ride a lot straighter than other 'reiners'. Some people think this lack of bend shows a 'stiff' horse. This is not true, either. He just rides 'straighter' when he is in the bridle. He rides with more arch in his neck, a vertical face and does not round his back as much as most high level reiners. He carries his head higher because his ultimate goal is to 'go down the fence' and work a cow at high speed. He has to 'look through the bridle' to do this and cannot be looking at the ground with his head between his knees. This difference has caused a lot fewer 'crossover' horses of late that show in both Reining and Reined Cowhorse.
Back to the Spade or the 'heavy bits' a finished horse shows in. The Spade has a brace above the lower part of the mouthpiece that looks like a copper spring. It allows movement of the cheek pieces. You can wiggle them back and forth. The more movement, the more 'pre-signal' a horse gets from the rider who is wanting to give it direction. Horses learn to carry a vertical face because the high ports and all of the heavy metal hangs on the top of the horse's head as opposed to lying on the horse's tongue. There is room for a 4" high port if the horse's head is vertical. That little bit of 'wiggle' is what the horse learns to listen to and then meticulously obeys the neck-rein.
Some horses work best with more weight, so some romal reins have 'slobber chains', some have 'weighted silver ferules' for more weight and others just have braided rawhide or latigo reins. I have picked up bridles with chains or weighted romals that weighed over 10 pounds.
Everything on a finish bridle and romal has a purpose. The 'purests' all ride with leather curb straps. The early day trainers I knew did not have a bridle in their tack room with a chain curb. They stayed in 4 rein set-ups until they had a horse finished enough to not require any 2 handed riding or any correction.
These finished horses are so light that a 'wiggle' of the reins or a change of 1 inch up, down or to either side with a rider's hand tells the horse all he needs to know. I have used a piece of light string to 'tie' a rider's hand to the saddle horn and give them less than 3 inches of string to move their rein hand. They had to be able to do this to be ready to show a bridle horse.