Man, I went to school for engineering for a while, and laminar flow and lift-induced drag became obsessions for me until I quit. I just typed three pages trying to get a concise answer across using explanations that are somewhat easy to grasp.
I will have to keep at my little paper and try to make some illustrations to help reduce the text, but I can sort of sum it up.
Glide is indeed a quality of flight based on it speed versus its ability to make lift. But there are a LOT of factors that explain everything from glide, to fade, to even high speed turn and how oat can cause the flight to change. The weird and cool thing about disc flight is that the math is calculated for the flight like you are doing math on an airplane wing that is the distance of the throw, that changes shape for the whole distance.
Now we know that discs don't accelerate more after we release them, but they can actually rise and lift in flight between the release and the landing. How much power it takes to generate that lift, and how much power the disc can handle before the lift is too high and it starts turning (actually stalling but really hard to explain that in short) is related to angle of attack in flight, stall speed of the disc at that angle, and lifting line of lift-induced drag imparted on the rear of the disc during the flight time where the forward speed is greater than required for maintaining a large amount of drag without stalling. (yeah it is pretty confusing)
A glidey disc that is understable has a very large angle of attack even when it is level, meaning it makes a lot of lift without much speed. It also has a high amount of lift-induced drag because its lift making ability.
This drag is made from the flow of air leaking from the bottom of the disc to the top (going from low pressure to high pressure area) around the back trailing edge of the disc. As long as the disc is not thrown too hard, this drag actually stabilizes the flight of the disc by giving a cushion of air long enough for the whole disc to ride on, with pressure underneath almost the same at front and back.
However, the slightest head wind, or too much speed at release, really bumps the front side pressure up, and leaves the back in very low pressure turbulent air all over. This actually makes the back of the disc rise thanks to rising line theory and causes it to turn down and to the right for the RHBH throw.
Conversely, when this type of disc does begin to slow down and fall, it has that big comfy cushion of air staying under it. This keeps it travelling forward more than down as opposed to an overstable or stable disc. The nose can't get much higher than the tail, which is what causes the disc to stall out and fall.
A glidey but stable disc has the same qualities as above, except it has a very aggressive angle of attack when level, so that when the speed increases, the cushion of air under the disc is enhanced by something I think is called a boundary layer of air, which is basically an envelope of smooth air over the back top of the wing made by extra turbulence on the front. This keeps more of the high pressure air under the disc and more low pressure over the top, with less lift-induced drag for the same amount of lift generated being the net result. So this type of disc plays a trick in flight by giving itself extra properties of stability based on inefficiencies it has at speed. We take advantage of this quality by over-powering the disc essentially to get it to fly neutral. Then once that disc slows down, it fades really hard, because that angle of attack up front stalls out the disc immediately as soon as the boundary envelope dissipates. Which is why us noodle arm guys can't huck a 175 tee bird as far as a valk for instance. We can't hit the speed of the heavy teebird necessary to get it to have that extra-neutral, "pocket" air layer built up. It takes a lot of speed.
Now onto stability. All discs follow the rules above at some speed. Even the most stable disc will eventually fly like a comet if you can torque it hard enough to get a low pressure pocket over the back of the disc. Net stability of the disc is derived from how much stall is occurring in flight over time and why. The same way good glide is a component of having a lot of lift drag which makes it less stable with more speed, stall from not enough speed prevents a more stable disc from flying longer unless you throw it faster. It can't use its angle of attack to make a even layer of air under the disc without speed, so the back of the disc falls, and since it is spinning, it fades out and to the left.
Now when we throw a disc, we know there are several ways to manipulate a particular disc to fly differently than explained above. Nose angle at release, hyzer and anh angle, forward speed, and spin speed.
Nose angle is easy to understand. This will allow the disc to break some of the rules of its flight before it starts to actually glide from stall due to leading edge resistance and the turbulent air from it. Since you have put more forward momentum into the throw than can be used for lift due to angle of attack, the disc is stalling, but it is still headed forwarded with more inertia than air can fight if you have a good snap. Your throw will yield a disc that essentially begins to turn over like you snapped too hard, and it will go right some acting like the disc with no pressure on the top of the back, because at such a low angle of attack they don't have enough resistance on the leading edge to make lift at all, only a big cloud of turbulence around the disc. Then the back falls again and they make the lift they should, but later in the flight.
Hyzer and anhyzer just modify the timing of when stalled flight or true glide takes over, similar to nose down but with less of an effect. The amount of spin affects how exponentially fast the disc slows down and changes flight characteristic. That gets super friggin hard and its late. Basically if you made a line from the center to one point on the edge of the disc, the more times that line points in the same direction for the same amount feet the disc flies forward, the more it will fly like an actually wing of it shape at every speed. So the more spin the disc has through the flight, the longer it will fly the way you released it.
