Lean thinking is integral to Agile, and the holy grail of Lean is flow. Flow makes knowledge and materials available when needed at each step in the value stream. Other descriptions of flow:
Steps flow smoothly with no interruptions, delays, or bottlenecks
Make the value-creating steps occur in tight sequence so that the product or service will flow smoothly toward the customer – Lean Enterprise Institute.
The Rock Crusher implements a straight forward flow equation:
WhatGoesIn = WhatComesOut (Thin Pipe + Waste Gate)
This equation highlights the rock crusher is not a reservoir of ready stories. The Rock Crusher’s funnel is intentionally larger than the thin pipe; after all, just because a rock is in the backlog does not mean it will be done. But leaving obsolete or forgotten rocks in the backlog plugs up the process and flow, creating friction and waste. Good backlog hygiene demands that we constantly remove clutter from the backlog. Unlike the stack of plates visualization, the Rock Crusher explicitly reveals there are two paths out of the backlog: the thin pipe or the waste gate.