I think someone one showed me how to do this, but I just can't remember. Here is my scenario: I have an existing feature class to which I added a couple of fields. Those fields are to be populated as a concatenation of some other field values.
What I did to run them manually was first set them for insert and modify. Then I selected all the features, and in one of the fields I have the new concatenation rules set, I simple hit the space bar while in the selected attributes pane. That's enough for a modification so both rules fired and I'm good to go.
However, that's sort of a hack; I thought there is a way to execute a rule manually without even making a phony modification.
That should just about do it....