Thanks M!
For those interested, the relevant code resides in rc/services.c under handle_notifications()
...
if (nvram_contains_word("rc_support", "nandflash")) /* RT-AC56S,U/RT-AC68U/RT-N16UHP */
eval("mtd-write2", upgrade_file, "linux");
else
eval("mtd-write", "-i", upgrade_file, "-d"...