Change the settings
If this is enabled, then users are allowed to request swaps for their duties. With a duty swap, the current assignee is responsible for finding a new volunteer.
If this is enabled, users are allowed to just decline an assigned duty, provided it is within a given time window. The duty goes back into the unassigned pool.
This is similar to the "free cancellation" option when booking accommodation and travel online.
The latest day before the duty when declining is still allowed.
For example if the duty is at 2PM on 30th June and this is set to 7 days, then it is possible to decline anytime up to 2PM on 23rd June.