Even the artist just says "an important part of its frame". If I had to guess, it's a bar connecting the back wheel's harness-thing with the gear to maintain a

Even the artist just says "an important part of its frame". If I had to guess, it's a bar connecting the back wheel's harness-thing with the gear to maintain a consistent distance. If someone sat on that, I think the wheels would flex out, the chain come loose, etc.