Hello David Hufford,
The Forward mapping dialog collects the Persistent model information from the source files and compiles the metadata. If you add a new class or make other changes to the model (remove,rename class/field etc) the Forward mapping would need to 'listen' to every single change made in the editor and update accordingly. This would considerably slow down the development experience.
Hence the 'Refresh' button. Refresh collects all the information available at that moment and again compiles the metadata.
Hope this answers your question.
the Telerik team