Once a Watch is created, it will scan artifacts in the specified resources when a scan-triggering event happens, and issue Violations accordingly. However, until a scan-triggering event happens, artifacts already existing in the system will not be scanned by the Watch. So, to make sure a Watch is immediately applied to the relevant artifacts, you can invoke it manually by hovering over it and selecting Apply on Existing Content.
Not available for All Repositories or All Builds
You can only manually invoke a Watch on existing content if the Watch is defined on specific resources and not on All Repositories or All Builds
Clicking the button pops up a dialog that lets you specify which of the resources assigned to the watch should be scanned, and a date range that defines when the artifact was last scanned by Xray.
For example, selecting Last 7 days will only scan artifacts that have been scanned in the last 7 days.