Starvation mode doesn't really exist. Sure you can starve, but that takes generally 72 hours of no Water and food for your body to kick into such a situation.
But what many people who attempt to lose weight eating extremely low calories experience is a metabolic downgrade, which means that your body adapts tot he low calories you are feeding it and when you exceed that number, you eventually gain weight, even though it seems silly to gain weight eating 1200 calories, but consider this.
A woman eats 700 calories a day and loses weight perfectly. However, she hits a plateau and wonders why. Well, instinct tells her to drop her calories even more, which may result in SOME weight-loss but at such low calories you're hardly able to eat enough food to fuel yourself, so then you generally opt to eat more, but adding to your calories causes weight gain because your metabolism has downgraded.
And the only way to really fix such a situation is to lose the insane calorie deficits and eat higher calories. I went through this personally, still am. When I was first starting out I was eating 600-800 calories a day for almost a good 5-6 months, but it worked, but it also ruined my metabolism. So yes I've packed on 10lbs, but my metabolism is re-regulating itself, so nowadays I CAN eat 1800 and lose.
Just something to consider.