The second photo shows how the cap is to be screwed onto the body of the Weed Whip head. The third photo shows the Weed Whip head fully assembled. That's all there is to it. Now you just screw the Weed Whip onto your trimmer, and you are ready. This is serious line.
Zundapp line is much stronger and thicker than ordinary weed line.
The Weed Whip is the first head that is designed right. It loads simply with your fingers in 30 to 60 seconds with no tools and no moving parts. You simply snap a new line into the grooves. The shoulder on the cap presses the lines deep down into the grooves. Ask anyone who owns one.