You are using Case Flags and would like the flag to be set / cleared through Workflow based on an action which is not available on the Case Flags Setup Wizard.
The Event Rules section on the Case Flags Setup Wizard allows administrators to decide how Case Flags should react based upon updates to a case. For example, you can configure Case Flags to set the flag when an inbound email is received on a case, or clear the flag when a new case comment is added.
Depending on your use case for Case Flags, you may want the flag to be set or cleared based on a specific update not specified on the Setup Wizard, such as when the value of a custom field is changed.
Case Flags comes packaged with a special field named "Case Flag Date/Time" which controls whether a flag is set or not. If this field is populated with the current time (or a previous time), the flag will be set. If this field is blank, the flag will be cleared.
In order to set the flag using Workflow, we suggest performing a field update on the Case Flag Date/Time field, setting it to NOW().
In order to clear the flag using Workflow, we suggest performing a field update on the Case Flag Date/Time field, setting it to Null.