Where a safe withdrawal rate targets a *high probability* of success, the failsafe rate is the amount that would have survived the single worst-case sequence in the historical record — someone who retired at the very worst moment (e.g. right before the 1929 or 1966 downturns). It's the most cautious number a fixed-spending retiree could anchor to.
It's deliberately conservative — it optimises against a once-in-a-century disaster, so most retirees will die with money left over. It's most useful as a floor: 'even in the worst history threw at anyone, this spend would have held.'
The planner surfaces the failsafe rate on the stress test page, alongside the fixed and flexible safe rates, so you can see the full spectrum from cautious to optimistic.