Warning, the text below may be entirely wrong for non-Korindo Aikido
As how I understand it, udekimenage should project the opponent forward or it will break his elbow joint while in the video it looks like breaking the joint was the intention rather than a projection. It left his opponent no way to escape in a peaceful manner whereas in Aikido there's still that option.
I suppose the question is what is the sensitivity required when using udekimenage that would prompt uke to be thrown forward instead of breaking his elbow. Too fast and strong, and it'll just be an elbow break, too slow, and the effect of the throw is diminished as he recentres and there's no real incentive for him to move forward.
You keep writing about forward momentum, but, if all the pressure you create is directed this way, your Uke will step forward to evade teh technique, without falling or rolling.
In order for Uke to fall / roll, the direction to escape from your break must be downwards. The way I learned creates a pressure that rotates in a circular manner along a small arc, first upwards and then downwards, thus creating Kuzushi and throwing Uke (or breaking his arm).
Hi Reuben, just from a theoretical point of view: one thing has to happen to make it require a roll as opposed to just a walking/jogging/hopping forward projection. That one thing is that the top of the body has to get accelerated forward, while the legs/bottom of body gets a lot less forward acceleration (or even gets acceleration in the opposite direction).
You pointed out one way to make this happen-- stop the uke's legs with your hip or your leg. Extreme version of this is judo-like throws or turning this into koshi nage.
In terms of basic mechanics of making it a throw instead of a forward shove or an "elbow attack," that will do it.
Another way to do this same top/bottom trick would be to manipulate the sensation of weight so that he can't step forward easily-- so there is some difficulty stepping, all the while his upper body is getting a lot of acceleration. I think the way to practice that is the common exercises of lifting (kokyu dosa or aiki age) as well as doing tai-no-henko with a sinking feeling. Still working on that. But neither of these threaten the elbow, it is just lifting and sinking, in ways that affect where and when he can easily step. So now when you go back to udekimenage, then you have something that is independent of the elbow.
Another way to get the legs moving kind of backward while you take the upper body forward would be a wave motion, maybe something to combine with the above method. In other words go back-then-forward, timed with the wave action in his body (this may be best with soft, flexible ukes), so that the legs are still experiencing "backward," while you have begun driving the "forward" upstairs.
Well, it's a forum so take this with a grain of salt! If I make a video or something then my "theories" can have more credence.
We might be writing about the same concept, hard to explain movement in words.
Perhaps the answer lies in this sensitivity. In putting enough energy forward to make a real opponent prompted to project and if he resists, then a broken elbow but not so much speed and energy to give him no opportunity to opt to roll forward.
It's like Uke has to choose either
a) broken elbow OR
b) forward projection
I suppose it's quite difficult to practice this in a dojo environment to see if it works in real life since nage has to be committed in his technique enough that if resistance is given, uke's elbow will be broken. If nage is still 'nice' about it, then there would not be sufficient impetus for a resisting opponent to fall forward.
In short, udekimenage is not a 'NICE' throw. It's you roll or you get injured. Compare this to iriminage or shihonage where the throw is inexorable and cannot be resisted once balance is broken but does not require any joint breaking for it to work. Here udekimenage has an element of 'u may be injured so you better roll'.
Thoughts on this?
Both iriminage and shihonage have variations which are "not nice", and we (beginners excepted) normally practice those, and not the nice ones. shihonage is a break and a backwards imbalance, and if Tori insists, Uke can not even fall to avoid the break (obviously we let go before that in our practice). In riminage, one has a powerful lock on the neck while throwing.
Aikido techniques are not unique, the same techniques exist in multiple styles of Jujutsu (often with lethal variations).