While the term 'starvation mode' is over used, there are plenty of people with experience that reducing what you eat has very diminishing returns. Your metabolism will reduce itself to match what is available.
Like an engine running rich or lean, it adjusts to whats out there. A similar example is water. When your body has access to plentiful water, you drink it, it pees it like no tomorrow, if your water intake drops it holds onto the water. If you eat less than you burn you will lose weight, in pretty much all circumstances, however the rate of loss can drop dramatically.
Most people have a sweet spot, if you follow the diet correctly you should be getting 300 calories from your syns, 240 calories from your healthy extras, and then a good 500 to a 1000 calories from free and superfree foods. There are weeks where i have found the pounds fly out, and other weeks, such as when i did a few success express, in which nothing was happening.
If you consume too little your body behaves weirdly, it holds onto all it can, it will also break down lean tissue such as muscle to obtain what it needs.
PS. For people with very high levels of fat, the above largely doesnt apply, your body is able to metabolise all the energy it needs from the fat cells in your body.