The clamp
Vanguard dynamic spending starts from a target percentage of the current portfolio, then refuses to let this year’s real spending jump more than a ceiling or fall more than a floor versus last year. The published teaching defaults are +5% and −2.5%. After a crash the target percentage would cut hard; the floor holds spending up. After a boom the ceiling stops a lifestyle leap.
That is the opposite shape of Guyton-Klinger. Guardrails do nothing most years, then make a 10% cut or raise. Dynamic spending fidgets every year and never makes a 10% jump. Neither is “safer” in the abstract — they move different risks. Rails put lumps on the lifestyle. Clamps put a slow bleed or a slow raise on it.
Run it
The Vanguard dynamic spending calculator is the page that actually computes the clamp. This guide is the mechanism. Compare it with Guyton-Klinger on the same portfolio rather than picking from a slogan. The nine-rule hub is the set.
When the clamp is the point
Use it when a 10% spending cut would be a household crisis and a 3% cut would not. Use rails when you would rather not touch spending until you have to. Use a fixed-real 4% when the spend is a floor you cannot cut at all — housing, a healthcare premium — and accept that the portfolio then holds all the risk.