Here's how I understand it:
A diet high in simple carbs increases the sugar in your blood, makin git spike.
Those of us with IR can't really handle that, and it gets stored as fat. Androgens (which cause things like the hairgrowth) is stored in fat tissues.
The more fat tissue, the higher androgen levels, the more hair.
Low carb will aid you in balancing your insulin levels, allowing your body to process what you give it better, help you lose weight, break up the fatty cells, releasing the androgens and decreasing them. You have less of the hormonal imbalance, which makes treatments (like laser) more permenant than they would be otherwise, but some women also see a decrease in the amount of hair growth they see.
This article sort of says the same thing, but in a more eloquent fashion:
http://sheknows.com/about/look/3038.htm