Easiest way to explain for me is for you to get down on your hands and knees. Now stretch your hands (forefeet) as far as away from your knees (hocks) as you can. No further than that. Right up to here as far as you can.
Now staying in that position. Lift both hands off the floor. This is how a horse goes naturally. It should be impossible to stay with your hands off the ground. Feel how much you would be working to try and do that for your rider.
Now (tap tap with my whip) staying in that position move your knees right up to your hands. NOT your hands back to your knees, as in dressage we do not pull back. NOW lift your hands off the ground. It should be dead easy. This is what happens with a horse when collected. See how easy that would now be to do for your rider.
So if a horse is being ridden correctly there should be no sweat on the neck. They have top muscle and no muscle on the bottom of the neck. The mane should be on one side of the neck. I am not talking about horses whose breeds have double manes.
Ridden incorrectly you will see sweat on the neck, again I am talking about a cold day and not like here at the moment, when my boys neck is sweating before I tack up, as it is hot and humid here. You will also see muscle on the bottom of the neck and mane on 2 sides of the neck.
As to the froth if that was the question. If you have wet hands on wet soap and move only a little bit back and forth you will not get much of a lather. This is a horse not using his back and legs just going back and forth.
Now same soap and wet hands and move a lot forward and back, up and down and round and round, now you have a lather. This is a horse using his back, his hindlegs are going back and forth and up and down and round and round with collecting, using his back and working.
My boy is a classic example of this. His thighs meet so you would think froth every time. Nope. Even when it is so hot. Nope.
This morning a trail ride and on the way home trot and canter on notch 2. Yes he has had years of the rule of only walking home and so now I can break that rule when I feel I can.
Into the arena and walk, trot and canter, collected and medium and over cavelletti and I had froth between his back legs. YAY.
OP should be saying YAY as well.