--- a/doc/Low-level-commands	Sat Feb 20 19:53:08 2021 +0000
+++ b/doc/Low-level-commands	Sat Feb 20 20:10:18 2021 +0000
@@ -162,3 +162,18 @@
 the osmo-auc-gen utility from Osmocom to run the expected algorithm with the
 expected Ki (and the expected OPc if MILENAGE is used) and the same RAND, and
 see if SRES and Kc match.
+
+Exploring proprietary APDUs
+===========================
+
+If the SIM you are working with is known or suspected to implement some
+non-standard or proprietary APDUs for which there is no explicit support in
+fc-simtool, you can use this low-level debug command to send arbitrary APDUs:
+
+apdu "xx xx xx xx xx ..."
+
+The sole argument is a raw string of bytes (quotes are needed if there are
+spaces between bytes), and the APDU needs to be given exactly as it is sent in
+the T=0 protocol: 5 bytes of header (including the length byte) followed by
+data bytes, if any.  After executing the APDU exchange, the apdu command simply
+prints the SW response code from the SIM.