@Snowcelt: I'm very serious. I don't practice HEMA at all-- why should I get a coach ? I write professionally about military history, so I keep myself informed about people who mess about with swords. I listen to reenactors, I listen to HEMA folk, I've held a shield, I've swung a sword to see what it's like.. But I don't put on breeches, and I don't put on the fencing mask. My postings in the very interesting fioredeiliberi.org forum are not those of a practitioner of that sport, but of a person who writes about, and reads about social violence.  
 
I do like to sling. So: if you do something with some interest, get a coach. This applies for most activities.  
 
Now the sticking point is: where do you get a slinging coach ?

It is to be coached very hard to themselves.
 
Often one does not note mistake, besides.
 
If one chooses, nevertheless, a trainer, one should also train after his instructions.
 
And today there a very big problem is hidden. Today information are to be got everywhere light. What is right, however and what is wrong?
 
So remains only one one must act. A final separation meet.
 
Then one is able to do a way goes.
 
After one year maybe, one checks his Errgebnisse.

Been musing over this "what is a coach" notion and have come to the conclusion that the concept is not fixed. When you start on some new activity a coach is fully an instructor who watches your every step and guides you. However the notion that a coach must always be better than you at any given activity is obviously flawed as that would imply that at the elite level only another top player could act as coach. As you progress, the coaches role changes from absolute teacher to careful observer, guide and encourager and even as a sounding board for ideas.
 
As an aside, one of our top Australian Rules football coaches just retired here after a 28 year career, and having taken a bottom of the table team to two consecutive Grand Finals. Yet as I recall he was no great light as a player in his earlier days and certainly at 56 years of age he wouldn't even be selected for a B grade team. But as a coach he will likely never be excelled.
 
So I think I tend to agree with Steven's assessment that the coach may not have to be himself an expert slinger but he needs to be a careful observer and understand the principles of human biomechanics so he can make posite suggestions about correcting inefficient actions etc. After all slinging is not some hypercomplicated activity, the movements are similar enough to tennis and other sports for a tennis (or other similar) coach to have meaningful input.
 
Also, in our case, we have the goal. Thearos wants to sling like Jaegoor as I understand it. Now Jaegoor's style is quite well documented; we have multiple video clips on YT even though most are rear views but we do have at least one good side view. So now all our hypothetical coach has to do is to turn Thearos into Jaegoor - simple. Well, perhaps not but at least we can go part way down the road.
 
Now, what are the obvious differences? Aside from the fact that both Jaegoor and C-A appear to a good deal more solid than Thearos which naturally gives them a very stable base, I can see see three things.
 
1. Footwork - the feet are not stable, rear leg slightly bent during initial rotation.
 
2. Excessive weight transfer onto the front foot during the actual cast which causes the "forward topple".
 
3. And, very importantly, the plane of rotation of the final thrust is not the same. Jaegoor's is a distinct downward chopping action with his right hand finising up near his right hip. Whereas Thearos tends to make his final thrust as an enlarged continuation of the initial horizontal rotations with his right hand finishing up high, approximately at ear level. Now I think that this is what draws him into the excessive forward weight transfer and subsequent topple.  The topple is not the problem in itself but a consequence of the final throw.
 
So were to from here? Coaches not only point out faults but work out programs to overcome them. So FWIW here's my suggestion.
 
Temporarily forget the sling. Get a length of rope or fold up some lighter cord to simulate the weight of a sling and stone. Now stand in your normal starting position, commence your initial rotations. I think it important to always do the same number as it builds up a routine, but I have noticed that Jaegoor and some others don't always follow this rule.
 
Now when you simulate the actual final thrust you must transition out of the horizontal rotations into the overhead, downward sweeping motion. (Here timing is critical which is all feel and something I cannot explain.) Do this by a partial weight transfer onto your front foot but primarily by a distinct body rotation, commencing with the hips and following with the shoulders. The shoulders should rotate through at least 120 degrees. Remember your body's centre of mass is somewhere around your navel so provided you keep it behind the front foot you should prevent the forward topple. Make sure your right hand finishes low and to the left of your body.
 
Do these repetiotions ad nauseum until it becomes second nature. Don't go for any sort of power, just a smooth flowing action. The hard part will be when you get out with a real sling again because you will want to revert to your old horizontal release. So at first don't try to hit anything, just sling to get the projectile going forward until your new style has become second nature. At this point I would try to get a video so as to check if your movements are really right.
 
I hope all of this will be of assistance. So hard to adequately describe actions in words.
 
Regards,
 
Aussie.
 

Thanks, Aussie.  
 
C_A-- many thanks, will think of it. Difficult with family obligations !

Sorry for bad English
 
 
Endscheidung.
 
 
It is important which one exactly knows what one would want.
 
If I want only fun. No problem
 
If everybody may have.
 
However, fun is relative
 

Aussie: my goal is to hit what I'm aiming at, not specifically to sling like Jaegoor. Nonetheless, i will give your programme a try !
 

Thanks, Aussie. I'm practicing what you recommend. But isn't what I was doing earlier just normal helicopter style ?

Got it, thanks. I always thought that helicopter meant release in the horizontal plane, but the angling as a matter of course is pretty good. Now to make sure that the shots don't plough...