-<directory name> should be a mount-point for this), and `zafio' (uses
-afio to compress each file as it is backed up). Only `dump' type
-backups perform incremental backups.
+<directory name> should be a mount-point for this), `zafio' (uses afio
+to compress each file as it is backed up), and `ntfsimage' (for NTFS
+volumes, requires device name). Only `dump' type backups perform
+incremental backups.
+
+<options> is a comma-separated list of <option> or <option>=<value>.
+Options supported:
+
+ gz[i][=<compressionlevel>]
+ Indicates that the whole stream should be compressed with gzip.
+ The compression level defaults to 1 if gz is specified by the
+ level isn't. gzi appliies only to the incrementals; gz applies to
+ both unless gzi is also specified. compression level 0 means not
+ to run gzip at all and is the default if gz[i] is not mentioned.
+
+ snap=<snapkind>
+ Indicates that the filesystem should be frozen before the backup
+ by using /etc/chiark-backup/snap/<snapkind>. See the head comment
+ in /etc/chiark-backup/snap/lvm for details of how this works.