I do not lose weight when I undereat. If you did that for a long time you eventually would (which is what people with eating disorders do), but then basically you'd be starving yourself, and it would be impossible to maintain your weight loss after that.
"Starvation mode" is what people usually refer to as your metabolism slowing down, which it can do considerably on a low amount of calories. It's a survival process: in the past food was not plentiful, thus during a drought in order to survive, we had to expand the less energy possible. No-one knows exactly how everything all works, but the general principle I tried to explain below - for example it was discovered recently that fat is an endocrine organ (= it produces hormones), so more fat = changes in metabolism yet to be explored and understood.
The advantage of having a high weight is that you get great results from eating more than you should but lose weight - e.g. if you usually ate 3500 calories a day, in order to lose weight, you have to create a calorie deficit. If you start eating 3000 cal a day then you'd probably lose weight. As you lose weight, you'd need to gradually lower down your calorie requirements until you reach a healthy balance.
I personally started losing weight by eating at least 2500 cal a week when I was overweight, as this was less than what I was eating at the time. As time went by I had to lower it, and to maintain at my height I need about 1400-2000 depending on how active (1400 sedentary, 2000 very active).
When you go from eating a lot, to eating too little (anything under 1200 for most people is "starvation" level - less energy than your body needs for "housekeeping", let alone daily activities), your body shuts down as suddendly it has considerably less than it use to, so tries to spare energy and adjust, considerably slowing your metabolism, to the point where you might not be able to shed the weight. If this is happening now when you're 270lb, then you really really want to eat more to avoid this at all costs.
Eating too little initially causes a lot of weight loss, which will be a majority of water and body waste. After a while of eating too little, you will lose muscle, further lowering your metabolism and making you unfirm. I've done it, and it doesn't look good.
I'd say eat healthy food as much as you can, don't deprive yourself too much, stay active, and allow yourself to have naughty food once in a while, may it be one day a week, or an allowance every day (e.g. I allow myself 200-300 calories of junk a day to keep me sane).
Good luck! I think my post may somewhat be confusing as I didnt write it in a logical way - let me know if I've confused you.