This property is used by the backup/restore system. Under normal circumstances, you do not need to set this property.
Yes if the enabled property of secondary keys will be restored during restore; otherwise, No. The default value is No.
During backup, secondary keys where the Enabled Property is set to a value of Yes are marked with a BackupKey value of Yes, and the secondary indexes are not stored in the backup. When the backup is restored, the BackupKey value is checked to see whether it reenables secondary keys where BackupKey = Yes, and the indexes are recreated.
You will normally only see the BackupKey set to No (the default), as it is only set to Yes during backup. If a backup restore is suspended during key creation (to be resumed at a later time), you can see and set the BackupKey property.