They usually ask volunteers - and they offer free ticket to the same class you are booked in if you offer to go on the next available flight. It depends on where you are flying from, if they have more than 1 flight per day - otherwise they put you up in one of the hotels and your meal is paid for as well.
If you not in a hurry to reach, it could be worthwhile taking the free ticket and bonus miles!
If no one volunteers, i guess then ppl who are skywards members get preference to fly or they could give more freebies ( like free first class tickets or more miles )to make the passenger take the offer.