I agree with you about Will and the rear foot. I feel the same exact thing in my FH, but clearly my foot has left the ground, it gets pulled/dragged forward behind the front foot so it's still countering the throw, but the rear foot is the main driving source to initiate the throw much like this:
Initiate yes. It is responsible for the last jump in lateral translation (horizontal) speed. But I've been talking about reach-back to release (or beyond), and even the submariner doesn't do much there.
The other force acting on you is acceleration.
Acceleration is not a force (it's the result of unbalanced forces, to keep it simple). The force of gravity is pulling us downward with constant acceleration right now. Earth pushes back. Forces are equal, no velocity change.
The purpose of the coaster's initial ascent is to build up a sort of reservoir of potential energy. The concept of potential energy, often referred to as energy of position, is very simple: As the coaster gets higher in the air, gravity can pull it down a greater distance. As the train starts down the hill, this potential energy is converted into kinetic energy and the train speeds up. At the bottom of the hill, there is maximum kinetic energy and little potential energy. You will weight more than your weight and feel very heavy at the bottom of the track as acceleration changes.
That's all well and good, but again, the disc golfer is not throwing downward, and if they maintain the same level from reach-back to follow-through, they've not used the potential energy they had when hopping.
It'd be like saying you should jump as high as you can, land, then do your usual run-up and find that you throw the disc farther. You obviously know that's not true.
The coaster tracks(your legs) serve to channel this force, they control the way the coaster cars(your spine) fall. If the tracks slope down(rear leg), gravity pulls the front of the car toward the ground, so it accelerates. If the tracks tilt up(front leg), gravity applies a downward force on the back of the coaster, so it decelerates. There is a ton of weight force felt from that deceleration and you must compress or thrust against it or collapse.
The amount of horizontal speed you can add by letting your CG "fall" (from gravity) during a disc golf throwing motion is negligible… particularly when nothing falls (drops, changes height).
So as your spine moves forward over the rear leg, the whole spine(center of gravity) falls forward and down as the rear leg guides it by changing the leg angle. Once the spine falls past a certain point of the rear leg, the leg must provide thrust against the ground to keep it from collapsing and drive the spine(cog) forward.
Again, the amount of horizontal speed you can add by "falling forward" is minimal, particularly since planting your front foot stops the "falling forward" process, and the rear foot spinning out or flipping up in the air puts an end to any and all ability to push with the rear foot. The rear foot becomes "unweighted" very early on in the disc golf throwing motion (around or just after the reach-back in many players throwing drives - obviously for stand-still throws they leave the foot on the ground often, but they also obviously don't push off much there either).
The front leg has to support your spine from falling over forward and transfer this energy and redirect it. The front leg either collapses not transferring energy through body, or it stiffens/straightens which is also a squatting/thrusting force. The angle of the front leg is not straight up/down either, it's angled like 45 degrees to catch your mass moving forward and down and clear the front hip. So the squat/thrust is as about as much vertical as horizontal(backwards), otherwise your front leg and spine would be collapsing. It's like you reached the bottom of the roller coaster track and it begins to go back up. You feel as much "weight" from the inertia of your internal organs being braced(stopped forward/horizontal) against the front leg as do from the earth pushing up vertically against the leg which is super juiced from falling.
Your leg isn't "super juiced" from falling. You've not only had almost no time to fall, you've not fallen very far at all.
The force you're feeling in the extending of the front leg is converted into rotational speed. It's a force generated by extension of the knee - your muscles. The horizontal speed you've built up by your run-up increases the force you feel in your foot (since your leg is, as you point out, about 45° or whatever - it's absorbing some of that horizontal speed, with the rest ideally continuing in your upper body to help propel the disc).
If there is no compression, or squat/thrust during this throw, then what is causing him to jump or spring up like a compressed spring in the finish?
Simply the fact that he wants to stand up and not fall over. You can be out of position when forces (rotational, translational, etc.) are acting on your body, but if you maintain some of those positions once you've stopped spinning, running forward, etc. then you're going to fall if your CG is not located over or between points of contact with the ground.
These pros are "springing up" WELL AFTER the disc has been released. What they're doing then is just to stand there and watch the disc. It has about as much bearing on the flight of their disc and the power put into the disc as when they sneeze two months later at home or not.
These guys are just re-gaining balance after their throws slow down and near coming to a stop:
http://cl.ly/image/3m1H3u2F3s3D . They're not "springing up."
Again, if you're talking about distance throws, for example, then it's a different beast:
http://cl.ly/image/35173w0s0Z08 .
In that image you can see he's still in the process of extending his knee, which will add force vertically, and primarily will really help SNAP (in a good way) the hips around ROTATIONALLY. It's the combination of basically jumping on his one knee (his right knee) and the momentum of his arm swinging upward that causes him to get into the air (his jump would be less impressive if he maintained a straight right leg, of course - his tailbone doesn't raise much, and his head goes up because he releases lateral flexion in his spine
http://cl.ly/image/2q1f2V3f0m18 ).
Val actually jumps up/forward through the hit similarly to Lisa Longball if she had more forward momentum:
I don't see it:
http://cl.ly/image/1k0x1j3l3X2H http://cl.ly/image/2z0Y3H3g3V2f .
If anything, I'd say she could probably add some rotational speed if she were able to extend her knee more, but that's the knee on which she has a brace, so that might be asking too much of her.
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My summary: I think you're attributing entirely too much to the role gravity plays in disc golf. Disc golf remains largely a horizontal (and a rotational) activity, and the amount that gravity adds to those by "falling" before you plant your front foot is negligible.
Obviously gravity contributes to friction, but disc golfers are not adding a ton of downward force for the purposes of increasing traction. What they have during their run-up is enough.
And again, obviously you feel a LOT of force in your front foot, because it's not only opposing (via friction) your run-up (horizontal speed) somewhat (since it's planted at 45° or so), but because you're actively extending your knee to help speed up rotationally. In distance throws, this snapping can also lead to a small leap, and to a little added speed to the disc because the launch angle is more vertical in a distance throw than a "golf line" throw.
I added these last bits in the edit window so I apologize if I was short or mistyped anything as I was in a hurry.