Starvation mode doesn't prevent starvation, it just slows the process, and it's most often seen in anorexia, bulimia, yoyo dieting, and in people who have experienced repeated or prolonged famine. Frequent periods of drastic calorie restriction (especially with very, very low calorie intake over long periods).
Illustrations of starvation mode - Lets take several imaginary women of the same healthy weight, BMI, and of similar body build and body composition. Amy has a long history of anorexia or bulemia, Beth is a chronic yoyo dieter whose diet of choice is diets drastically under 1000 calories. Connie has had gastric bypass and has lost 300 lbs, and Denise lived through years of famine, and Elaine has never dieted and has always maintained a healthy weight without calorie restriction.
Remember, they all are starting with the same amounts of body fat. Now, put all of them on a no-food fast and see how long it takes these women to die of starvation (and assuming none of the women damaged their hearts during their experiences with super low calorie dieting), odds are that Elaine who has never dieted will lose weight most quickly and will starve to death, the quickest. Those whose bodies have endured the most and longest periods of starvation diets will tend to live the longest.
For another imaginary example, let's say identical twins of identical weights and weight histories decide to lose 15 lbs. One uses 1000 calorie diet, and the other a 500 calorie diet. If "starvation mode" is all fiction, the second twin should reach goal in half the time of twin A (assuming neither cheat). Odds are that twin 2 will lose more rapidly than twin 1, but not twice as rapidly. It's also wouldn't be impossible or unusual for them to lose at a similar rate (especially after the first few weeks).
There comes a point in calorie restriction when fewer calories isn't better. If that weren't true, the best weight loss strategy would be eating absolutely nothing until the desired weight loss is achieved. That tends not to work very well.
I have experienced several of the metabolic effects commonly described as starvation mode. I would say that most have developed slowly, and over the course of years, if not decades of yoyo dieting and yoyo eating and starvation diets (0 - 500 calories).
It hasn't made me weightloss or starvation proof, but I am probably goning to lose much more slowly than someone my weight and age who has never dieted before.
I think the only thing most people need to take away from the idea of starvation mode is that weight loss is achieved by eating fewer or burning more callories, but there comes a point where fewer isn't better. 300 calories per day probably isn't better than 500 for most of us, which probably isn't better than 800...
As to when less stops being better, that isn't always easy to determine. I think it boils down to eating as much (healthy food) as you are able to while losing weight at a rate you find tolerable. Don't cut calories to the point that you risk sever muscle loss or cardiac damage...
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