I'm gonna quote myself here from an earlier comment:

As a part of a midfield 3 but plays a lot more as a second striker. Balls aren't hoofed to him unless from Courtois or if the opponents are pressing high. The main difference is that Wilmots, while he plays Fellaini as a midfielder, he knows he'll go up up front and so does the team, Radja and Witsel come deeper and De Bruyne/Mertens (whoever plays RW) comes more inside to stop a fast break in case Fellaini loses the ball. Which he does a lot, don't get me wrong, but the system makes it that we get possession back faster and don't get hit on a counter so it doesn't matter as much. When he recieves the ball there's also always a player near him so he doesn't have to try any hard passes.

This system is not only around Fellaini though, all the players have to react to another's actions and playstyle. The problem is that he's trying to make it that all 10 players react to eachother which causes some chaos from time to time. This would be a good system if he centered it around 5 players. The 3 up front and Radj+Felli.