I spotted this interesting bit of software on Hack a Day. The basic idea is to do a hard reset and boot a Linux environment, which can then dump the contents of the computers RAM which will still contain any encryption keys and decrypted documents the user was working on.
It’s an old idea, in fact I used to do it when ripping music on the Amiga. Wait till it’s in memory, reboot and then scan for mods in RAM. However, there is a fairly simple defence too, which also worked on the Amiga.
All BIOSs offer the option to do a full memory check. That involves writing to every address in RAM and reading it back, which of course would overwrite any remnants of data left there. On many BIOSs there is no way to skip or disable this action before it has happened (typically you can only access the BIOS menu after the RAM check) and it would certainly not be hard to develop a BIOS module which would be impossible to disable.
On the Amiga a similar trick was possible using the Soft Reset Vector, which allowed you to run any code you like at reboot time.