What-Ifs are overlays. They don't edit your saved-simulation inputs. They sit on top and perturb the engine's view of the world for one re-run. Toggle them off and your baseline numbers come right back.

How multiple toggles combine

Most scenarios combine independently. For example you can run Equity Drop at Retirement together with Capital Gains Increase and the engine applies both. The one exception is the inflation pair: Inflation Higher and Inflation Spike are mutually exclusive (only one can be non-zero at a time), since they describe different shapes of the same underlying assumption.

When multiple scenarios are active, they're applied in the same order every run, so results are deterministic. Adding a second scenario doesn't undo the first. Both effects stack.

Reading the diff

Once a What-If is active, the Proof view shows the new success rate and ending-balance distribution under the modified assumptions. The summary band at the top of Proof flags that the run is What-If-modified, so you don't accidentally read a stressed result as your baseline.

To get a clean side-by-side, save the baseline as one simulation, duplicate it, apply your What-Ifs to the duplicate, and load both into Compare Sims. That gives you real two-column visibility. The donut and the summary table for each, lined up.