Chain-driven. That's nice, most of the stock Harleys are belt-driven, which I always thought was a poor choice for transferring a lot of torque. Not sure why the guys has rear pegs, but no rear seat.

The other part I'm not fully seeing is how this is a decisive or serendipituous moment. Seems to me it's a simple shot of someone's butt.

Sorry but I don't see a moment here : although a great shot.

No relevance for me. This exposes or reveals nothing about what it is to be human, which is what Bresson, as I see it, was after. That's quite the machine, though :)