How does the second spool load on reel?

[ccices 0]

Is it supposed to snap in somehow? It just appears to hand on the other holder. It looks like it should click in someplace but I can't seem to figure it out.

[tmostad 5]

It doesn't snap in. It is held in place with the arm that guides the filament. Not a super positive feel to retention but it works.

BTW - Due to the way the arm is designed it matters which way the filament is wound on the spool of some spools. For example, the spool design plus the direction of wind means you'll have to rig up some way to use Matterhacker's Pro filaments. I don't know about the others. It would have been nice the design was more universal.