V64 is a popular Swedish betting form for trotting, with races several times a week. To win you must pick the 4,5 or 6 winning horses. If no one picks all the winners or the payout on 4 or 5 winners is below 7 SEK, the winning pot (for e.g. 5 winners) is transferred to the next V64-race and the added to the winning pot for 6 winners.
If your horse is a non-runner, you will get a reserve horse instead. Either you could chose it or it will be selected automatically.