We've been assuming that Felix fixed his fastball by modifying his mechanics. The guys on MLB network brought up a really interesting point. When Felix reported to spring training this year he was thinner than I had ever seen him and he looked like he could be a model. Look at him since he has gone on his 12 game tear: he has put weight back on, and apparently he said that he needed to do so to get his fastball back.
So there you go, he didn't need to adjust his mechanics, he needed to ramp up his Twinkie consumption!