There are plenty of studies out there that prove this to be true. There are studies that simply have a controlled group switch to a lower GI diet with no other changes and end up losing fat mass. There are studies that show certain supplements(CLA is one) to cause fat loss and muscle gain with no other changes in diet/exercies. Then there are studies that show how people in other regions of the world, specifically China in one study, actually eat more calories on average than Americans yet manage to stay much leaner and healthier.
I know John Berardi at T-mag has a few articles from last year that are entitled "Lean Eatin" I believe that cover a lot of what you're asking. I'd suggest giving them a read as they may answer a lot of your questions.
As far as exactly how it works, well there is a lot of hypothesis out there for why things work and anyone who says they have all the answers are lying. I'd say to simplify it just think of a 24 hour period as a many microcycles. Most people clump weight loss or weight gain based on the period of an entire day. That is to say at the end of the day either you've gained, lost or maintained weight. Most people think that you cannot simultaneously gain muscle and burn fat, ie; be anabolic and catabolic at the exact same time(this is of course not counting anabolic assistance). This may be true, however this doesn't mean that you can't switch between these modes many times during the course of an entire day which would lead in time to changes in body mass.
When you wake up in the morning your blood sugar is low, glycogen stores are low and you are in prime fat burning mode. Say you do some cardio on an empty stomach and manage to burn some stored fat. Then you eat a few meals that are basically maintenance level calories, and let's assume that they are "clean" as in low GI, moderate to high protein with some healthy fats. Then perhaps in the afternoon you perform a resistance workout. You may actually burn some fat during this time as well, though it doesn't really matter per say. Afterwards you take your post workout shake and then eat maintenance level the rest of the night. Since it's post workout your body is shuttling all the carbs into the muscle and as long as fat intake is low you have no risk of fat gain.
Now that you've provided some incentive for your muscles to grow your muscles will begin to repair themselves and start to use that protein you've been eating. Hopefully you've taken some protein before bed so that you can continue to allow your muscles some fuel to rebuild. Assuming you have eaten some protein then you may actually gain some muscle that night, albeit it a very minute amount.
Jump to the next day and your muscles are still repairing themselves, therefore greedily using the protein you're intaking to do so. The micro-trauma itself is actually causing your metabolism to be elevated(heavy eccentric exercise causes this even more) somewhat and assuming you're still eating maintenance level you really aren't giving your body a chance to store any fat. This is also assuming you're eating a clean diet and keeping your blood sugar stable during the day.
I think you can see over time this type of situation could stimulate LBM gains while slowly dropping bodyfat. Of course this is not an optimal way of doing it as it will be extremely slow. Not only that but you have to keep your diet in check at all times. Then think of other possible methods. Maybe you eat less than maintenance calories the first half of the day and hopefully burn some fat. Then you perform your workout and eat more calories for the second half of the day to promote some LBM in that time. Try to focus carb intake around your workout as your muscles will greedy suck them up and you'll have far less chance of changing them to fat. Then you take the next day off from the gym but reverse the caloric intake time periods. Start out eating slightly more than maintenance for the first half of the day to promote some more LBM from yesterdays workout, then switch to lower calories in the evening to perhaps burn a bit of fat, maybe even adding some cardio before bed.
These are just some basic ideas but I think you can see what can be possible. Nutrient intake timing as well as proper nutrient makeup at your meals can make a huge difference in what type of weight you end up gaining or losing. If you manage to keep up this type of lifestyle consistently then I think you will no doubt begin to see change, it just won't be as drastic as if you went on a full fledge diet or bulking cycle.
At any rate I'm just throwing out some ideas for you to ponder. It is possible to do what you're asking, it will just take longer to do both together than either one at the same time. If you're patient and just want to be healthier while changing your body comp for the better then I think that this type of regimen may be good for you. Take the right supplements(check animal's site for the anarchy stack) and do your workouts and cardio at optimal times and you'll be golden.
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