Ritalin -- and other stimulants -- work the SAME WAY in people with ADHD and normal people. They do the SAME thing. If you take speed/Rit/whatever, you will notice that your
ability to concentrate on things is vastly increased. For example, normally I can barely be bothered to keep my dirty clothes in a pile, but tweaked out, hell, I can concentrate on cleaning the house for hours...
Seriously, the difference is that people with ADHD have a natural deficiency in their 'stimulation' or 'reward' circuits. The signals that their brain sends them saying "pay attention: this is important!" just don't get through[1]. In some people, this makes them unable to pay much attention to anything -- the 'inattentive type' of ADHD. In others, their brain compensates and tries to amplify the "pay attention" messages. But since messages don't get through, there's just a lot of background noise that gets amplifies, so these people's brains jump around telling them to pay attention to pretty much anything. This is like your bf, the 'hyperactive-type' ADHD.
Stimulants like meth, ritalin, etc act to fix this by (properly) amplifying the pay-attention signals. In normals, the same thing happens as in ADD people: if you take a low dose of stims, just your brain's actual signals get amplified, and you'll find you can concentrate better on the things that interest you. If you take higher doses, the stimulant starts to amplify background noise too, and you can find yourself paying attention to random, stupid things, becoming obsessive about them, jumping around flightily, and so on.
Make sense?
Here's a good article I read on Ritalin recently:
http://jama.ama-assn.org/issues/v286n8/ffull/jmn0822-1.html
[1] Dougherty DD et al (1999). Dopamine transporter density in patients with attention deficit hyperactivity disorder. Lancet 354:2132-2133.