Keto diet quesgion

My husband is always on diet. He is counting calories at the moment which seems to work for him.  Our friend told us about "keto diet" and that he starts to do it. 

Can someone who used this diet tell me if it is really work?? You eat meat, fat, butter, cheese and a lot more and do not put weight on?? How is it possible??

Can you eat potatoes or pastas on this diet or you replace it somehow?? 

I am really curious about that diet. Seems to good to be thrue :)