Process flow: how to force conditions before changing a picklist

[Guido1982]

I want to perform a small test-task in the process flow management, since I've never used that before. Now I know we can enforce certain conditions using the process flow, so here is my testcase: I have a dropdown that can only be set to a certain value if a numeric field is set to zero. Is there documentation on the process flow where I can figure out how to do that?