It's a slightly difficult one to explain, but I'll try. When you start an FM match, it plays the match through in order to determine what the key highlights are, in order to show them. This is done on-the-fly - for example until a goal is scored, nothing knows the goal is going to be scored. It's done by playing out all the events, player positions, interactions on the pitch using 'weighted' random numbers - the 'weighted' being the key bit, that ensures that better players by and large out pe