None of the 777 expert talking heads I have listened to for the past 6 days have even mentioned the AHM let alone how it works. I am an aerospace engineer and experienced pilot. I did not know about the AHM until I started google searching AHM and ping yesterday.
If they could turn it off, an ideal time for a hijacker to do so would be after the first 2 pings and right before the last ping to make everybody think it's crashed. Some of these terrorists are pretty crafty.