i'm pretty sure the 50g is just a guidline; the actual amount of carbohydrate would depend on the person. but, basically, to stay in ketosis you need to be eating few enough carbs for your body to use an alternate fuel source to glucose. it's not just carbs you have to watch though. protein can be converted to glucose through gluconeogenesis, which can also knock you out of ketosis.
i wouldn't spend much time fretting over the specific numbers. if you want to burn fat by going low carb, fill up on high fat and moderate protein and let your carbs come mainly from veggies, and your body will get more efficient as using bodyfat stores for fuel.