Sorry, I posted again after I watched the video... and still didn't get it. My spool was out of reach pretty quickly, and I'm too thick to see why having the double-ender would help. Either it's in my hand (with the spool and line down below) or I let go and the double-ender slides down the line to join the spool. I'm just being stupid, I guess, and am probably missing the bit about having a loop in the line or something.
EDIT: Now I sort-of get it. the double-ender keeps going down at the same time as the spool, and after the line has run out the double-ender keeps going down and brings the spool back up to your hand. Sounds great, as long as the spool down't weigh more than the double-ender, right?