It is known that Scrum is based on Lean because the document that defines Scrum, The Scrum Guide, explicitly states, "Scrum is founded on empiricism and lean thinking. Empiricism asserts that knowledge comes from experience and making decisions based on what is observed. Lean thinking reduces waste and focuses on the essentials."
There may be people who disagree with what constitutes Lean, and who is a thought-leader in the realm of Lean-thinking, however, I will assert that Eliyahu Goldratt was one of them. He was the originator of the Theory of Constraints (ToC) and the author of a dozen books related to continuous improvement in mass manufacturing. A central metaphor to one of his books entitled, "The Goal" was Drum - Buffer - Rope, scheduling process focused on increasing flow and throughput.
The "Drum" is a symbol of steady cadence, which to a mass manufacturing line can be expressed as takt time. "Buffer" represents an accumulation of materials, work orders, or input enabling the smooth progression of adding value through a system. The Buffer absorbs variability inside and around the work system. "The Rope refers to the mechanism that controls the flow through the entire process." as the DevSoc article explains.
In Scrum there are 5 Scrum events which occur on steady cadence. The Sprint is a container event with a duration of 1 month or less. There are 3 Scrum Artifacts in Scrum. One of them is called the Sprint Backlog which is a buffer, and the Increment is another buffer. When a Scrum Team is practicing single-piece flow inside of their Sprint, they are essentially placing a Rope metaphor into their work system (which also contains a Definition of Done) to increase the likelihood of creating value amidst variability, uncertainty, ambiguity and complexity (VUCA).
I want to stress the fact that Sprints in Scrum are carefully design to yield value in conditions entirely distinct from those of mass-production factory floors. Scrum succeeds amidst VUCA to produce novel, completely unprecedented end-user value incrementally one or more times within a Sprint. Unless and until all of the elements listed in the Definition of Done are rigorously and transparently verified to be completely satisfied, a separate item in the Sprint Backlog is not undertaken by anyone on the Scrum Team.
Even in the case where no such practice as single-piece flow is in effect among a Scrum Team, the Metaphor of Drum - Buffer - Rope applies to Scrum Teams. The Buffer is again, the items which the Scrum Team pulls into their Sprint at the Sprint Planning Event. (Ropes can only be pulled, never pushed successfully. Goldratt stressed the indispensible characteristic of "pull systems" to ToC.) By the Scrum Team making a voluntary and informed choice of how much work to pull into their Sprint, overburden is avoided, and risk of non-delivery drastically decreases.
The Sprint is a fixed (unvaried) length, the cadence of which is symbolized by the drum. Regular, uniform cadence allows humans to instinctively anticipate when the next boot will drop, the curtain will fall, or the work will culminate into value to inspect, learn from, and improve upon in future Sprints.
I might go so far as to say that the summation of all the Sprint Goals into the Product Goal (showcased within the Product Backlog) is a type of Goldrattian Buffer. At the Product Owner's sole discretion and explicit designation, multiple Increments produced by the Developers of the Scrum Team in single or multiple Sprints may be accumulated into an individual release, which fits the profile of yet another Goldrattian Buffer, though it is not an endemic, parameterized constraint from the Scrum Guide. Rather, the Product Owner adapts her plan for release to the continuously emerging realities of the market the Product serves. One could rightfully say, the release timing is variable and "Empirically optimized."
The Rope symbolizes more than pull systems alone. It also is a mechanism that controls flow. Each Sprint has both a Sprint Goal and at least 1 usable, shippable Increment. The Rope ties together everyone on the Scrum Team to focus their efforts and attention on no other work that fulfilling the Sprint Goal within the cadence of the Sprint. The Rope binds them to the Sprint Goal, as the focus of their combined work efforts.
Every Scrum Event has it's own "Time Box" which is on cadence, and each has its own clearly defined and agreed purpose, which is the Rope pulling the focus of the participants in the respective Scrum event together. The growing, evolving, increasing artifacts and data points are the Buffer to each event, as are the various, valuable outcomes: learning, decisions, experiments, and new or modified Product Backlog Items.
Without the enabling constraints represented by Drum - Buffer - Rope, which in Scrum are the time-boxes and cadence of Scrum Events, combined with the Sprint Goal, Sprint Backlog, Definition of Done and the cross-functional consistent membership of a small, long-lived Scrum Team, waste and delay would occur in the process of creating the Increment(s) of the Sprint.
The ability to forecast and circumvent any kind of waste including overburden or delay (to say nothing of the 8 Lean Wastes) is enabled by constraints. The Focus which arises by working within the confines of the Drum - Buffer - Rope constraints is called a "Scrum Value" and there are 5 of them: Focus, Openness, Respect, Courage, and Commitment. The Scrum Values support the people who take on themselves the enabling constraints mentioned above.
Yeah, but so what?
Applying systems thinking is critical to success of the Sprint. Without a very firm understanding of the interrelatedness of the enabling constraints, with all of the best of intentions, someone might propose or insist on removing some of Scrum's elements. Tampering with the system called Scrum is ill-advised. So, Scrum Masters, trainers, and supporters of Scrum would be wise to actively share information about why Scrum works, help others see it as a stable work system too, and continue with a Beginner's Mind to inspect its operation, and uncover deeper awareness how the system dynamics impact longer-term outcomes.
Eliyahu Goldratt was a visionary thought leader who substantially informed the revolutionary thinking which Scrum is based on, and continues to elevate the human condition. I celebrate his life along with thousands and perhaps millions of business people and new product developers world-wide.
Thank you, Mr. Goldratt!