I'm not sure, each time I've not been needed (they've even ended up clearing standbys on each flight), so I've always flown the flight I purchased from the origin, it's just the onward points that have been cancelled because I didn't show up for the 'backup' booking that they held while we waited to see if I was needed as a volunteer to be bumped.

Also, this is the offer being accepted, I almost always put one in, this is when I'm called up to the gate, they confirm that I am willing to be bumped, and they hold the later flight / alternate itinerary. I've never had an issue when I put an offer in but never speak to anyone (the so-called 'data collection' portion of this)