For people with ADHD, amphetamines basically erases the symptoms of ADHD and decreases hyperactivity, so it doesn't really act like an upper at all (although it does still increase alertness).
ADHD is caused by a low dopamine level or uptake. This can lead to concentration problems and impulsive behavior, and those symptoms can make people look restless and overactive. Amphetamine stimulates dopamine production, and that cancels out the effect of ADHD. If given in a low enough dosis to someone with ADHD, it takes the restlessness away, so it has the effect of a downer instead of an upper.
