Can you tell me how can amphetamine reduce hyperactivity in people with adhd?

These meds have: A paradoxically calming effect on those suffering from adhd. See http://www.Drugabuse.Gov/publications/drugfacts/stimulant-adhd-medications-methylphenidate-amphetamines for a complete explanation of why these stimulants work the opposite way with those who suffer from adhd.

In short: in short, think of the stimulant medications as "stimulating" the brakes of the brain (inhibitory controls of the prefrontal cortex).

Brains underaroused: Brains of children with ADHD ae underaroused or less active than normal. Amphetamines increase the brain level of nor epinephrin, making it more alert and the children less like overtired 2 year olds.