Default Update.zip question...

Hi forum...

A friend of mine told me that the desire hd was able to update firmware from a update.zip you can put on the SD card...
How do that work ?
I can't seem to find any usefully information on that...
He said that if there is a update.zip on the SD, then the phone will flash it at boot...

Could that be my way back to a Root-able firmware ??
If yes then how do I get such a update.zip for my phone ?

Sent from my Desire HD using XDA App
Regards PsHdO