Double lines, for mowing and thinning work. Mowing lines are automatically extended when the mowing head is tapped on the ground.
This is a rapid feed head that allows you to fill it with nylon line by twisting the center spool. When setup correctly you feed 4 meters of line through the eyelets so that 2 meters are on each side of the head. Then when you rotate the center spool it sucks the line into the head automatically.
This is a much more efficient way of loading your trimmer head.