The thin pipe is a metaphor for the team’s limited capacity. Push too many rocks or excessively large rocks through, and the thin pipe will clog and block flow.
The team pulls rocks through the thin pipe to deliver a solution increment. How the team creates a solution increment is beyond the scope of the Rock Crusher. The thin pipe simply represents a point at which the team could commit to pulling ready rocks through the subsequent steps in the value stream, such as development or fabrication.