I’ve seen this being done on a few Holga vids as well. Although mostly with the older Holgas, The new ones have a tighter fit I think..plus that rubber padding at the back.
I was surprised how tightly rolled the last roll was wen I opened the back.
It just sits under the bottom of the film spool.the one with the unexposed film that is.
Found this vid via a discussion in the Diana users group on flickr, good tip for keeping the spools tight.
There is a heap for Holga on youtube too, most of them seem to be about loading the film though, not much about keeping them tight. I’m sure there are some, somewhere…