In a multiplayer game, this combo prevents everyone beyond three players (you and the people sitting to your left and right) from getting a turn.
Relatively simple, you activate Aeon Engine at the beginning of the turn of one of the people sitting next to you. When it gets to your turn, you activate Karn's -2 to get Aeon Engine back, and then activate Contagion Engine to replenish those two counters you spent on Karn. Then you cast Aeon Engine again to activate it during the turn of the player sitting on the opposite side to you. This requires 9 mana each turn to do.