Works best with backing and laying on Betfair-
If you place your qualifying bet on an event that finishes after the event that you place your bonus on (e.g qualifier on premier league game,bonus on horse race tomorrow),as long as your selection for the bonus bet loses,then you have no rollover.
If the bonus bet wins then your back to square one, but it's handy tip to have a chance of beating the rollover requirements and extracting as much money from the free bet when laying it off.
Interesting. This would definitely allow you to withdraw your money from their site? I guess it's been tried before?