+.TP
+\fB--update-of\fR=[\fIWIMFILE\fR:]\fIIMAGE\fR
+Declares that the image being captured or appended from \fISOURCE\fR is mostly the same as
+the existing image \fIIMAGE\fR in \fIWIMFILE\fR, but captured at a later point
+in time, possibly with some modifications in the intervening time. This is
+designed to be used in incremental backups of the same filesystem or directory
+tree. \fIIMAGE\fR can be a 1-based index or name of an existing image in
+\fIWIMFILE\fR. It can also be a negative integer to index backwards into the
+images (e.g. -1 means the last existing image in \fIWIMFILE\fR).
+.IP ""
+When this option is provided, the capture or append of the new image will be
+optimized by not reading files that, based on metadata such as timestamps,
+appear not to have been modified since they were archived in the existing
+\fIIMAGE\fR. Barring manipulation of timestamps, this option only affects
+performance and does not change the resulting WIM image.
+.IP ""
+As shown, the full syntax for the argument to this option is to specify the WIM
+file, a colon, and the image; for example, "--update-of mywim.wim:1". However,
+the WIM file and colon may be omitted, in which case the WIM file will default
+to the WIM file being appended to for append operations, or the WIM file from
+which a delta is being taken (only if \fB--delta-from\fR is specified exactly
+once) for capture operations.
+.TP
+\fB--delta-from\fR=\fIWIMFILE\fR
+For \fB@IMAGEX_PROGNAME@ capture\fR only: capture the new WIM as a "delta" from
+\fIWIMFILE\fR. Any streams that would ordinarily need to be archived in the new
+WIM are omitted if they are already present in the \fIWIMFILE\fR on which the
+delta is being based. The new WIM will still contain a full copy of the image
+metadata, but this is typically only a small fraction of a WIM's total size.
+.IP ""
+This option can be specified multiple times, in which case the resulting delta
+WIM will only contain streams not present in any of the specified base
+\fIWIMFILE\fRs.
+.IP ""
+To operate on the resulting delta WIM using other commands such as
+\fB@IMAGEX_PROGNAME@ apply\fR, you must specify the delta WIM as the WIM file to
+operate on, but also reference the base WIM(s) using the \fB--ref\fR option.
+Beware: to retain the proper functioning of the delta WIM, you can only add, not
+delete, files and images to the base WIM(s) following the capture of a delta
+from it.
+.IP ""
+\fB--delta-from\fR may be combined with \fB--update-of\fR to increase the
+speed of capturing a delta WIM.
+.IP ""
+As an example, consider the following backup and restore sequence:
+.IP ""
+.RS
+.nf
+(initial backup)
+
+$ wimcapture /some/directory bkup-base.wim
+
+(some days later, create second backup as delta from first)
+
+$ wimcapture /some/directory bkup-2013-08-20.dwm \\
+ --update-of bkup-base.wim:-1 --delta-from bkup-base.wim
+
+(restoring the second backup)
+
+$ wimapply bkup-2013-08-20.dwm --ref=bkup-base.wim 1 \\
+ /some/directory
+.RE
+.fi
+.IP ""
+However, note that as an alternative to the above sequence that used a delta
+WIM, the second backup could have simply been appended to the WIM as new image
+using \fB@IMAGEX_PROGNAME@ append\fR. Delta WIMs should be used only if it's
+desired to base the backups or images on a separate, large file that is rarely
+modified.
+.IP ""
+Note: unlike "pipable" WIMs (created with the \fB--pipable\fR option), "delta"
+WIMs (created with the \fB--delta-from\fR option) are compatible with
+Microsoft's software. You can use the /ref option of imagex.exe to reference
+the base WIM(s), similar to above.
+.IP ""
+Additional note: \fB@IMAGEX_PROGNAME@\fR is generalized enough that you can in
+fact combine \fB--pipable\fR and \fB--delta-from\fR to create pipable delta
+WIMs. In such cases, the base WIM(s) must be captured as pipable as well as the
+delta WIM, and when applying an image, the base WIM(s) must be sent over the
+pipe after the delta WIM.