They can assign you recovery flying from open time as long as they assign it prior to 12 hours before your originally scheduled report time. If you don't have anything on your schedule by 12 hours prior, then you are under no obligation for recovery flying and you can WS trips and get paid for both your dropped trip and the WS trip. Any recovery trip cannot start earlier than the original trip, but it can end 4 hours later than your original trip. You get no hotel if you get an uncommutable recovery trip.

It can be awesome if there is nothing in open time, but when there are plenty of trips sitting there it usually ends up being a bad deal.