Your Shotokan move is basically the same move. You are seeing the Uechi version of it. It's all the same. Study this style long enough and you'll understand the subtle differences. That fits in to the way things typically are done in Uechi vs. the way they are done in Shotokan. But it's like speaking English with a Bostonian vs. Texas accent. It's still English, and it still communicates. The differences just tell us where you came from.
There are so many variations of the position that I'm hesitant to say "the salute means..." I've heard people talk about touching a chi meridian just above the eyebrow, blah, blah, blah.
We do things open-handed because we like to grab and gouge in Uechi. Thus hands must always be ready to... We are more frontal because that's the way we fight and that's the way you'll be when in bad-breath range. We have the upper arm closer in to body and forward because of our penchant for being on the inside and doing things "smaller." The upper arm is on a slant vs. straight up and the lower leg is slanted the way it is vs. tucked in because of the kind of "slipping" we like to do in Uechi. Arm rubbing teaches us how. This explains Uechi's "accent" vs. the one in the painting above. Sanchin's principles dictate the Uechi Ryu "how" of this motion. If you had never seen the move before but had studied Sanchin from me, you'd likely end up with a very similar final posture.
Willy has the right idea here. There are myriad ways to use good movement, and he just showed you a really good one. But throw someon in a yet-to-be-determined scenario, and you'll find a completely different use for the motion.
Kata are a study of human motion. There's a biomechanical logic about the movement the way standing and punching like a white belt has biomechanical logic.
Just as when you thrust out with one hand and chamber with the other it creates complimentary action-reaction motions within the body, so to does a heaven-and-earth movement create a sort of biomechanical logic. What happens when you are pushing down with the left hand here is that the reaction would cause your body to lift. The upper movement can serve as the counter force for the downward movement - with or without it actually grabbing something.
Many grappling moves involve causing big things to happen by moving someone in a "smart" way. One way is to rotate someone about their center, which is a lot easier than actually moving their center from point A to point B. The easiest way to do that would be by grabbing someone at 2 points equidistant from the center, and then "turning the wheel." Pick any point on the body away from the center, and the other hand can find a spot 180 degrees from that. Then do the heaven-and-earth thing. The body will move. That then creates "opportunities" for you to do something with that extra leg. It could be hitting with the knee, it could be catching someone with the hook of your flexed ankle and dumping them, etc., etc.
Aikido actually has something called a "heaven and earth" throw. But it doesn't have the attitude of the extra leg.
It should even work when grappling on the ground. Everything is the same but your orientation w.r.t. mother earth. I would even argue that groundfighting will offer you MORE opportunities to use the move than upright fighting. One example in forms which confirms this is a way I learned the movement in a crane form. In Uechi you jump back on one leg, standing upright. In this crane form, you jump back and duck down on one knee. It's different, but then it's the same principle applied at another level - literally in this case.
Rory is the kind of guy who thinks like this, and can show you countless ways to use a simple principle in grappling.
So when you think about it, you should be able to come up with an infinite number of possible applications just by applying the principles I explained above.