The position of the dirtbag.
Explanation: This joke plays on the double meaning of “dirtbag.” In a Hoover vacuum cleaner, the “dirtbag” refers to the actual bag that collects dust and dirt. The punchline humorously implies that on a Harley Davidson motorcycle, the “dirtbag” is referring to the person riding it, using “dirtbag” as slang for an unpleasant or rough person. The joke works by subverting expectations and using the word in a different context.