9:23 It isn't a theory. A smaller wheel driving a larger wheel reduces the final speed but increases the force. The motor drive wheel (not visible here) is small and drives the larger wheel (the one you are touching at 9:10), so the larger wheel turns slower than the motor. The small, inside wheel (at 9:10) turns even more slowly, but with more force, and drives the upper wheel, again still slower and, again, with more force. So, the end result is the machine runs at a slower speed but there is more power at the needle. This could all be accomplished with just one very much larger wheel driven by the motor but imagine how that would look compared to the compact size of this solution. It's just another example of the wonderful design of these machines. Engineers may scoff at my explanation but I think I'm pretty close here.

Apologies for a comment not related to this video but I’ve been pouring through your videos for the past 24 hours or so after discovering your channel. My teenage daughter inherited a 1941 Singer 15-91 in a Singer table with stool (almost exactly like the one you reviewed 3 or 4 years back) for her sewing projects. I’m currently working through basic lubrication and adjustment for her use. I just watched your “bulldozer” video and discovered you are (or were) also in the Charlotte, NC area. Any advice on local parts/tool outlets (bobbin case, bobbin tire, jeans needles, etc) or estimates on refurbishing this reasonably functional machine? Thank you!