When creating models for FlightGear, you can choose from two physics engines—Yasim and JSBSim. JSBSim takes a set of functions that simply define arbitrary dynamics model, which allows fine-tuning the behaviour, but requires a lot of data that are hard to derive if you don't have wind tunnel or at least CFD data. On the other hand Yasim takes dimensions and some elementary performance data and guesstimates all the values, but the result can't be tweaked and any effect it fails to simulate can only be added by modifying the engine.
So this tool tries to replicate the guesstimation done by Yasim, but instead of running the result directly inside FlightGear it produces JSBSim model definition that can be tweaked and have additional effects added by post-processing or manually.