I can't tell you of any real resources that give much detail into it. I've always been told its just a known trick. Back in the day it was the way to create the ability for people to transfer a caller directly into a users embedded voicemail box. So you'd end up with a *Ext Hunt Group for everyone in the system, all call rerouted always to voicemail.
The way it works, to my understanding anyway, is that voicemail will see the * in the digit string and essentially escape from the mailbox of the device that routed the call to voicemail, and then evaluate the EXT portion, and route the caller to that voicemail box instead.