Yup if you Volunteer, you get a credit on the spot. It will print off on a boarding card (and and email will be send with the same information as well). It's a credit that can be used towards base fare only for up to 1 year.
If you don't volunteer, and get bumped by not your choice you get the choice of "cash" or credit. If you pick the cash, I "think" a cheque gets mailed to you.