If you want to learn more about how a low carb diet works, I suggest you read about the Atkins diet (high protein, high fat, low carb). There is a lot of information on the net about Atkins and this information will apply to low carb diets. A low calorie diet causes a state of ketosis.
When people eat less carbohydrate, their bodies turns to fat for energy, so ketones are generated as fat is broken down. Some of those ketones (acetoacetate and ß-hydroxybutyrate) are used for energy; the heart muscle and kidneys, for example, prefer ketones to glucose. Most cells, including the brain cells, are able to use ketones for at least part of their energy. But there is one type of ketone molecule, called acetone, that cannot be used and is excreted as waste, mostly in the urine and breath (sometimes causing a distinct breath odor).
People confuse ketosis with ketoacidosis. Ketoacidosis occurs in insulin dependent diabetics and is fatal if not aggressively treated in a hospital setting.
The Atkins diet recommends only 20 grams of carbs for the first two weeks to cause ketosis. After that, 40 grams of carbs are recommended.
When someone is ketotic they may feel miserable. Some of the symptoms are as follows:
Tiredness or fatigue
Feeling thirsty all the time
Metallic taste in the mouth
Nausea or stomach ache
The Atkins diet has now fallen out of favor because of its extreme carbohydrate restriction and high fat content. There are some medical issues which may occur with Atkins and low carb diets
Here is an article from WebMD about the Atkins diet and medical concerns which are applicable to low carb dietshttp://www.webmd.com/diet/atkins-diet-what-it-is
If you restart carbs and the total calories you consume does not exceed the amount you burn up, you will not gain weight. It is impossible to gain weight unless you caloric intake exceeds your caloric needs.