Fixed spending means drawing the same real amount every year, come what may. Flexible spending means adjusting — trimming discretionary costs (travel, dining, hobbies) after a bad run and topping them up after a good one. Real retirees are naturally flexible; a single fixed number hides that.
Why it matters so much
Most plan failures come from being forced to sell assets while they're down early in retirement (sequence risk). Flexibility breaks that: by spending a little less in the bad years, you leave more invested to recover. In the historical stress test, switching from fixed to flexible spending often turns a plan that fails most downturns into one that survives most of them.
The catch is how *far* you'd really cut. The stress test lets you set the flexibility (from 'cut to the bone' to 'won't cut') and shows how many historical eras survive at each level.