I'm using only for a selection box. I didn't want the user can select more than one field. If he does it; the selection box won't select anything.
I have a selection box using my field %SERVICE. so it display me all the services and the user can select one or more services. Now, i'm going in the menu Parameter-->Propriety of the document. In the Triggers, in the middle: "Event field triggers" I will select my field %SERVICE and push on the button "In case of selection" something like that (sorry i don't have an english qlikview). So I will add an action: "Selection in the field", it's when the selection is made. There is 2 fields to complete: Field and Search In. In field of course you will write your field: %SERVICE and in Search in: here for limit the selection to one, you will write the expression: =Only(%SERVICE)
I used it for this, hope it helped you to understand how you can use it for your problem