-You can read notes about the GNU S2K extensions in DETAILS from GnuPG,
-which you can fetch this way:
+GnuPG's [DETAILS
+file](http://cvs.gnupg.org/cgi-bin/viewcvs.cgi/trunk/doc/DETAILS?root=GnuPG)
+describes this extension this way:
+
+ GNU extensions to the S2K algorithm
+ ===================================
+ S2K mode 101 is used to identify these extensions.
+ After the hash algorithm the 3 bytes "GNU" are used to make
+ clear that these are extensions for GNU, the next bytes gives the
+ GNU protection mode - 1000. Defined modes are:
+ 1001 - do not store the secret part at all
+ 1002 - a stub to access smartcards (not used in 1.2.x)
+
+And [`gpg(1)`](http://linux.die.net/man/1/gpg) says of `--export-secret-subkeys`: