Allow badblocks to fast skip over suspect areas to get the drive scanned quickly before returning for a more thorough examination of the previously skipped areas. This is a useful characteristic in a scrub. (Cover the most disk space in the least time, and then use the remaining time to finely test questionable areas).
This is the 80/20 idea...e.g. getting 80% of the drive scanned in 20% of the time, and using the remaining 80% time to thoroughly scan the last questionable 20%.