system
Given the following post, respond as if you're commenting on Reddit:
human
[ELI5] How does Adderall (amphetamines) calm down those with ADD/ADHD. It seems counter-intuitive that stimulants calm someone. Explain! :D

Title says it all. Just doesn't add up. I've been told (and experienced) drinking coffee or anything with caffeine wires me for sound. How does it have a reverse effect?
gpt
There is a part of your brain that assesses what's happening to you and around you and basically portions out your brain's resources (focus, memory, retention, etc) to it based on importance. A leaf falls off the tree in the backyard? Barely even registers. The dull hum of the computer in the corner? Your brain tunes it out. But a gunshot goes off outside? 100% alert, full power to everything, what the fuck was that, get up and focus right NOW. A movie you're watching is halfway along that scale.

ADHD is, to oversimplify, a disorder in that system that means your brain constantly underestimates how stimulating things are. With severe ADHD, no amount of stimulation or urgency is enough for your brain to dedicate your full attention to it; important things get tuned out and your brain is always feeling laggy, irritable, looking for something to stimulate it. The football game you're playing is as important to you as a dull novel is to an ordinary person, and a dull novel is as important to you as the barely-audible sounds of traffic outside.

Have you ever been so bored that you're just thrashing around, pacing up and down, frustrated? You've got your taxes to do, but they just make you so much MORE bored that you can't even sit down and do them, it makes you space out and the words just don't register? That restless-legs feeling in your mind? That's what it's like. Being bored for a long period of time makes you really antsy and hyper.

What stimulants do is make your brain assess everything as much more important. So for an ordinary person, they would think "Okay, nice, this music has got some energy to it" and bop their head, then take speed and think "FUCK YEAH, this is the BEST FUCKING SONG EVER", and dance for hours. Everything is jumped up in urgency 50%.

So if you're ADHD, you take stimulants, and it pushes your brain's urgency levels *to what would be normal for other people*. Everything seems 50% more energetic and vital and important to your brain, so now it actually gives out your attention and brainpower and focus the way you need it to. 

This is actually calming, because before your brain was desperate for any kind of stimulation and constantly saying "Nothing's going on. Dude, let's go find something. There's nothing here. Go do something else. No, nothing important here either. Nothing's been interesting in days." With stimulation it will say "Okay, this is important, let's turn on memory and focus and pay attention to this", and you can work and feel satisfied in what you are doing.

If you are ADHD and take *large doses* of stimulants you get the same overhyped wired feeling that ordinary people get from a medium dose.