going briefly over what has been written so far...
what differs from strong arming vs. someone "throwing with arm" is that strong arming implies that the necessary fluid bio-mechanics do NOT happen at the right time (or at all).
the greatest feature of strong arming is that more power in = less power out.
throwing with a relaxed arm helps some of the natural timing mechanisms to happen. it can help all of the ones necessary ones to happen assuming that initially they are not.
for people not throwing 350', but pretty much listed the most common culprits:
1) Not pulling tight enough
2) Not stopping the elbow
3) Nose up throws
4) No wrist extension
5) Pulling too early
6) Not weight forward
7) Not finishing
these people will have a set of drills to get them to a certain baseline, which is a consistent 350-410' line drive power with modern technology.
the reason i am making this stipulation is because many of the recommendations given so far will work for someone throwing below 350', but are not really applicable to those who are throwing that far or farther.
i have had approximately a 1% success rate with teaching people how to truly "hit it." more like 4% if you consider "hitting some of it" (some of it = 430-450' with a wraith/destroyer).
for those chasing the brass ring, here's a few things:
-legs do less than you really want to believe they do. i have a feeling many people think that leg power is responsible for a large amount of throwing power. it is responsible for roughly 5-20%. you can meet someone with craptastic footwork/leg power that knows how to hit it and they'll be able to break 400' with a 1 step throw. huge run-ups, 360 turnarounds, etc. do add benefit, but it's not by a huge margin. however, if you are throwing in a distance contest, you want to use the technique that will milk out every potential bit of power. piss poor footwork can ruin a throw, but great footwork doesn't ensure a good throw.
-disc golf throwing is the one inch punch. to throw really far, you must throw really hard. knowing when/how to deliver force is the key of this. most players decelerate entering (and through) the power zone and the end result is a slip (even if it goes straight and decently far). hitting it requires acceleration through the power zone. for those unfamiliar with dv/dt, basically it means: as you get closer to the rip, your hand (and the disc) must be moving FASTER than it was at every point before that. you have a better chance of hitting it if you enter the power zone at 30mph and reach the rip at 40mph than you would if you entered the power zone at 60mph and reached the rip at 50mph. i developed the right pec drill to attempt to isolate the power zone while still using full body motions.
here's where things get tricky...
-there's a 2 stages of extension (in other sports these are similar to releasing the club head or releasing the barrel of the bat, but because joint release will get confused with disc release, i will call them extension). the abrupt stop of the elbow moving forward allows the forearm to extend (with whip-like inertia). at some point the forearm can no longer move forward and it rapidly changes direction from forwards to sideways... at this point the wrist extends (also with tons of inertia).
the difficulty in timing occurs because:
-the forearm/elbow must be relaxed at the beginning of the extension but should be firm/strong near the end of the extension.
-the wrist/hand must be relaxed at the beginning of the extension but should be firm/strong near the end of the extension (and subsequent release of the disc).
the reason you teach all the other crap outside of just this is because people need the coordination/skills to have all the other pieces in place in order to allow this to happen correctly. you can prevent these things from happening with poor body positions, but again, good body positions don't cause these things to happen, a better way of putting it is: good body positions ALLOW for the correct things to happen.
if you look at any/all of players who throw 450'+ line drives, they all have the same important things happening. i find it less useful to contrast differences than to look for similarities.
since a few players forms have been cited here, i will state this now:
there's only 3.5 styles of throwing out there, but players that hit it with any of those styles have all of the important factors in common. there will be hybrids between the styles, but overall they can be described within these terms (even a hybrid is just say "a mix of style A and style B").
the two primary styles (i'm making these terms up right now):
1. "American" technique.
2. "Swedish" technique.
the main difference between these two styles is the focus of the power base. american technique uses the kinetic biomechanics to generate its power, with the ideal being elbow extension. swedish technique uses much less elbow extension but tons of leverage on the outer edge of the disc.
Brad W.'s idea of throwing a stick/hammer vs. throwing a disc pretty much describes the primary determinant of swedish power. swedish power is based upon being able to lever the shiz out of the edge of the disc opposite the hand and find a way to translate the absurd amount of angular velocity/acceleration as the disc pivots out of the hand during wrist extension.
the current swedish technique derives heavily from Tomas Ekstrom's form.
american technique also uses leverage, but it is not dominated by leverage in the same way.
american technique can basically be broken down into 2 primary categories and one subcategory:
1. bent elbow (and derivations of it, probably most commonly found in carolina)
2. long reach back.
2a. folded shoulder rotation.
2b. spinal axis rotation.
2a. and 2b. both derive power in the same basic way, but the presentation of it differs slightly. however, most folded shoulder throwers are hyzer dominant and most spinal axis throwers are anhyzer dominant. in truth, pure hyzer mechanics use a folded shoulder and pure anhyzer mechanics use a spinal axis, what makes these two differ enough in styles to be noted is how they throw during a FLAT throw.
as for bent elbow vs. reach back, it's probably best compared to the differences in a wrist shot vs. a slap shot in hockey.
bent elbow throwing is efficient and it's easier to hit all of it. if you flub it, you flub it bad.
reach back throwing is less efficient but has greater power potential if you hit all/most of it. it's easier to flub it, but the diminished output during flubs depends on how bad you flubbed it.
the big thing is that long throwing bent elbow throwers and long throwing reach back throwers (as well as long throwing swedes) have more in common during the important parts of throw than they have differences.
the main differentiation between reach back throwers and bent elbow throwers (even those that use some reach back) is this:
-with a reach back throw the angle formed between the upper arm and the shoulder/chest collapses at the start of rotation and extends entering the power zone.
-with a bent elbow throw the angle formed between the upper arm and the shoulder/chest is constant at the start of rotation and doesn't extend until well into the power zone.
my point in writing all that?
what happens with the disc/arm during the final 12" of the throw is exactly the same for everyone throwing 500'.