rules & theory
Take turns drawing a line between two neighbouring dots. If your line closes a box, you own it and move again. When every line is drawn, whoever owns more boxes wins.
Near the end, drawing any line hands your opponent a chain of boxes. A strong player takes all but the last two, then gives those two back. That keeps control of who has to open the next chain. This is the double-cross, and Amog on Magnussen level knows it. He also solves the endgame exactly once it is small enough.