For whatever it may be worth, after I began training seriously (when I did such a thing) I experienced this (or something similar, at any rate). The first time I noticed this I thought I was bleeding from my pores, but it wouldn't wipe away. I just chalked it up to increased circulation...which is, I believe, an example/attribute of increased ki flow, right?
Also, I remember being told my first day of class that I should consider the feeling of pushing a shopping cart while getting groceries; which resonates with the phrase, "when one thing moves, everything moves," and suggests to me something about how one might begin to approach moving from center, making the arms extentions of it instead of pushing with arm strength.
Let's just take that phrase, "when one thing moves, everything moves". This does not mean "moves in space", as in your arms are rigid sticks affixed to a body that is moving, such that the arms get pushed along in space like they were just that, dead sticks. This does not mean your arms are limp noodles being whipped around as if they were injected with botulinum It means they actually move, as in they articulate, relative to their parent body. Dead sticks is no ki. Limp noodles is no ki.
The problem with arm strength is not that you shouldn't use it, just that most people's body, minus the arms, is actually in its totality a dead stick or a limp noodle that doesn't help at all, actually detracts, with the action of the arm just pushing them away or pulling them in, rather than acting on what its trying to act. This would be exemplified by the mindset of dps's example, "Have someone pull you by your belt. Your center will move first.", i.e. your body has just collapsed into a wet noodle or you have move yourself like a giant stick lever, the better to push you over with, either way assuredly disconnecting yourself from the floor, so what's left for the arms if you can't even get the ground up to your legs?
With the shopping cart, pushing it away from you as quickly or as fast as possible might be a better example. It's light, so no problem right? If any part of you moves back away from the cart during this, that was a collapse, a bleed, an energy blockage. If one part of you maintains fixed just to ride on the momentum of the other parts, same deal. Even if it happens for an instant and is later (over)corrected, same deal. "When one part moves, all parts move." Now try the pushing with something heavier, a car would be perfect, but we don't always have cars to push around. If you can get the car to move without anything collapsing backwards away from the car, maybe you're getting closer, so long as you tried to move everything, and the car in effect held them in place, not you. Subtle point: if your feet skid backwards, that's a collapse, sending force in the wrong direction, backwards, not down. The only thing that moves away is the car, not you.
Absurd example to point this out, imagine you had a little elf sitting on your shoulders around your neck. Call him Mr. Armsly
. If all you do is carry him around and he just sits there doing nothing freeloading off of your movement, he's a dead weight, may as well be carrying a sack of potatoes, little more than a glorified elf-potato-sack club (or flail, if you prefer). If Mr. Armsly starts falling backwards off your back, and he tries to push something, he will have nothing to keep him from falling further, and every time he pushes, he will in fact pull you backwards. If he falls forward and starts bending your neck over, and he tries to push something, all his push just goes further into collapsing your neck and pulling your farther over forwards. If Mr. Armsly falls onto one of your shoulders or the other, he similarly starts pulling you over off. Mr. Armsly does his job precisely best when he is sitting directly on top of you, all his pushing going straight down through your structure, I daresay your skeleton, your spine, maybe that is even a "center" of sorts, but that would be reductionist. Mr. Armsly has a tough job to balance there, and in fact has to actually actively use his legs and hips to grasp onto you while he pushes or else he will feel guilty if he accidentally pulled you over. If you move ahead of him, or let him move ahead of you, it basically causes the same thing. The more he goes at cross-purposes with you, the more you two are not acting as a team. Likewise, he hates it when you give him nothing to do, because he fancies himself more than a potato sack on your shoulders. He really wants to act as a team, but you keep making it so hard. In fact, sometimes Mr. Armsly feels you are too prideful, too erect in posture, for him to actually do his job well, and that sometimes you two both need to compromise on your stuffy upright positions to best get the job done, there is no I in team! Maybe he needs to work forwards, but you are telling him you only want to go straight up - there's no way you two could cooperate. Round off those sharp edges in the disagreement, bend a bit in your resolve to bring your two disparate directions in closer unison. Take one for the team.
Most days I feel like Mr. Armsly is real and he needs some ritalin. That's to say nothing of Mr. Legsly. Sometimes they decide they're having a party and Spiney is not invited, and then all hell breaks loose. Probably more useful thinking of helper elves than going looking for red spots or warm hands. But I digress.
And that gets back to, who made up the moving with the center thing anyway? Perhaps an oversimplification overglorification of that whole hara thing the Japanese seem so obsessed with? I don't think it was O'Sensei, that's for sure. Oh sure, he mentioned a hara, but did he really say it was a single point or that it operated in isolation? I really don't know the answer to those questions myself. I am no scholar or historian. But, are we putting too much stock in one, perhaps even misguided, interpretation amongst any other cool ones we could create that might work better, or just as well? Hmmm. What's aikido? What's not aikido? Probably doesn't matter at all anymore, so long as it improves performance in the thing.