As you unscrew the bolt with the proper hex key (8 mm) it will get loose then tighten up again as the bolt comes up against the "outer piece" and begins to pull the crank. Keep turning and the whole thing will come off.

One hint on reinstallation. Unscrew the outer piece so you can see that the splines on the spindle properly engage the corresponding grooves in the crank arm. When your sure the installation is correct, replace and torque the fixing bolt (don't forget the washer) then screw the outer piece back in. A dab of grease on the outer piece threads and the bolt head will make the next maintenance easier.