Dana, first of all I own an Iwata HP-CS and it's a brilliant airbrush and you have definitely made the right choice.
The adjustments you are talking about are done by a combination of altering the air pressure on your compressor, the thinning ratio of your paint and your trigger finger. For instance, 12 psi, highly thinned paint and a light pullback on the trigger = fine detailed painting.
As cbreeze said practise is what you need and the easiest way to do that is just fill the paint cup with water (no cleaning afterwards) and blast away at some old news papers. A double action brush sounds like a lot of work but it's not, it just takes a little adjustment.
If you want to shoot me through an email I'll send you a PDF file (in english) that shows how to care for your airbrush.
Oh and one last thing, when you are taking the nozzle, needle and head caps off the AB for maintenance, remember to draw the needle towards the front of the brush so that any paint on the needle won't go into the body. Oh and make sure you loosen needle chucking nut before removing the needle.
Happy modeling
Darren