I think that when insulin levels are low during a reduction of calories then more muscle will be preserved--less muscle will be used a fuel source.
A reduction of carbs usually yields lower insulin levels. Low GI carbs can also yield lower insulin levels. If you restrict yourself to eating Low GI carbs(excluding your post workout meal), which are usually relatively low in calories, then you will almost inevitably be eating "low carbs." A modest fat intake (around 30% from calories) may also have it's benefits, but I'm not sure if this is the case...maybe by increasing utilization of fat for fuel. I'm sure others will come through will a better explanation, as I have forgotten how this works.
Others will give better reasoning, but the main reason I eat around 25-30% calories from carbs and 30-35% cals from fat is because I have read that it may preserve more muscle than calorie ratio's higher in carbs and lower in fat.