Wireless Joint

dfu= device firmware update, it is the same as download mode that these Sierra devices use(your 6400 is a Sierra device) 9008 mode is the mode known as emergency and is the mode made for wiping/writing from 0 to the end 7ff if it is a 4k system or 1000 if it is a 2k system, your 6400 is a 4k system ...