By under control they mean, "Make it never happen." The same can be seen in in Frozen. Now there's a lot to be said wrong about the way the parents, and trolls, acted in Frozen.
Trolls: I know, we'll mind wipe her so she'll have no idea why you're afraid you might hurt her and thus keeping your distance and therefor think that she's been completely abandoned by one of the people she loves most.
King: Good idea, and I shall do everything in my power to make sure that both of my daughters grow up emotionally broken.
But the point here is not how well meaning people can completely fuck things up.
The point is that there's this idea "Under Control" means making it so that the thing never happens.
Bottle it up inside, never let it out. Build a dam with no way through, when it starts to over-top because of the ever growing lake behind it, don't open the floodgates --you never installed floodgates-- just make the dam even higher.
All focus is on the damn dam.
Under control means make it never happen.
We see a similar thing in Marvel's the Avengers where Captain America is convinced that Bruce Banner so much as letting off steam will mean the end of the world.
Tony Stark has a different idea, don't let it all build up inside until it finally boils over, let it out in a smaller controlled amount.
But the Avengers never really comments on who was right, which is why there's no need for spoilers for it here.
But the point is, whether you're Bruce, Elsa, or Vanellope under control means repressing. It means suppressing a part of yourself, it means don't do that, full stop.
There's a reason that people have seen a connection to, say, being gay here. "You've got to get these homosexual impulses under control," does means, "Stop fucking having them." Of course being gay is not a superpower (that I know of, please send all contravening evidence to cpw at maine dot rr dot com) and it doesn't put other people at risk of hulk-smash or ice shards of through the heart if you lose control of it.
So under control means make sure it never happens.
But then something happens.
Vanellope finds herself in a situation where she'll die if she doesn't glitch. For Elsa it's this:
[note that I have no sound on this computer, I'm just hoping this works]
Couldn't keep it in, Heaven knows I tried.
Don't let them in, don't let them see.
Be the good girl you always have to be.
Conceal don't feel, don't let them know.
Well, now they know!
Let it go, let it go.
Can't hold it back anymore.
Let it go, let it go.
Turn away and slam the door.
I don't care what they're going to say.
Let the storm rage on.
The cold never bothered me anyway.
It's funny how some distance,
makes everything seem small.
And the fears that once controlled me, can't get to me at all
It's time to see what I can do,
to test the limits and break through.
Suddenly "under control" takes on new meaning. Elsa has her power under control when she makes her ice palace, well under control. It's beautiful and sturdy and fracking awesome. If Disney isn't selling miniature versions for outrageous prices then Disney has forgotten how to make money.
Vanellope is able to make her way, unscathed, through an enemy horde, I'd call it an army but they were somewhat worse than that, steals a car, and rescues Ralph, when he's confused how she got to him she simply replies, "Don't worry, I got it under control."
And she does. We never see her accidentally glitch again. Instead she directs it into healthy pursuits, like saving her friend. Under control stopped meaning never doing it and started meaning controlling where, when, and how you do it.
Elsa takes a bit longer to get her powers fully under control. Her emotional story is more difficult, her psyche more broken, but by the end it's clear that it's by using it, by letting it out, that she gains control.
Under control stops meaning, "Make it never happen," and starts meaning, "Make it happen when you want it, how you want it and then it'll stop happening when you don't want it to."
It would be like if someone tried to get their movement under control by remaining perfectly still all the time. It would never work. At some point some part of their body is going to say, "Fuck this," and spasm. But if instead the person tried to control their movement by moving, perhaps one day they could be a dancer or a fighter or any number of other things where being in control of when you move and how you move is important.
Under control stops meaning, "Don't be yourself," and starts meaning, "Be yourself the way you want to be (because if you try to suppress it it might start leaking out in ways you don't like.)"
It stops being, "Never get angry," for the Hulk, and starts being, "Let off steam so that you don't explode when things get too much," of course that assumes Tony was right and the movie doesn't address that.
That's a theme I could do to see more of. Not, "Don't do this thing," but, "Do this thing in a healthy way."
And I could have sworn I had an actual point, but I lost track of it.