A GM will encounter many different events in the course of play, and must know how to resolve any given injury at any given time. The rules are specifically cinematic, so he may just base his decisions on whatever he feels would seem the most likely thing to happen in a story rather than basing it on reality or some difficult mathematic equation or convoluted rule system. A GM should know these rules, but does not necessarily have to live by them. If they are well-versed enough in the rules, they should know enough about the game to be able to bend the rules to fit every situation. The thing we want to avoid the most is the GM and players saying "I'm not sure how best to deal with that... let's take a twenty minute break while I consult the rules."
Remembering to be fair and not attempting to kill the characters off at every turn, you will need to be ready for anything. What will you do if the characters are in a car accident, or if someone drops a brick on them out a twenty-story window, or if a Surgery roll receives a Critical Failure? What if someone develops a laser and fires it at someone, or if a nuclear explosion goes off? We can’t possibly give rules for everything, and so the GM must always be alert in order to have the most realistic gaming experience possible. Especially important will be the result of called shots. If, for example, a character tries to throw a knife at a gun-wielding opponent's trigger finger, how will you deal with it? Will the gun be dropped? The finger be severed? If so, what type of a penalty will be applied for having lost the finger? What happens when a character's liver is damaged? Kidneys? Intestines? Lungs? Spleen? Situations like this call for a good understanding of the game rules and a creative imagination, both of which are important aspects of any GM.