The simulator is primarily derived from the data-driven 2018 U.S. House Midterms Election Forecast from The Crosstab by G. Elliott Morris. However, it also takes into consideration the consensus projection of a number of long-time expert forecasters. This has the effect of limiting extremely unlikely outcomes, as there is an extra hurdle districts considered 'safe' by all the experts must overcome before they will change parties in a given simulation. However, the net effect of this 'governor' across many thousands of simulations is relatively small.
We're hoping to add a feature in the days ahead that will list all the district flipped when a simulation is run.
* The simulation occurs before the map is populated, so this choice doesn't affect the outcome