Two retirements can earn the same average return and end very differently, purely because of the order the returns arrive in. A big loss in the first few years — while your balance is largest and you're withdrawing — does lasting damage, because you sell assets while they're down and they're not there to recover. The same loss late in retirement barely matters. This is sequence-of-returns risk.
The historical stress test
The planner's stress test replays your plan against every major downturn of the last century — 1929, the 1970s stagflation, the GFC, and more — retiring you *straight into* each one. Instead of random returns, it uses the actual sequence of returns that followed, so crashes and their recoveries land exactly as they did in history. You get a survival scorecard: how many eras your plan lives through.