GEOM_SHSEC is one of the less frequently used GEOM modules from FreeBSD, but it is actually pretty interesting. It combines several drives (or any other entities which are presentable as GEOM devices - including USB memory keys and files) into a single virtual drive which has the property that all its constituent devices must be present and available before its contents can be accessed.
For example, you might have two USB keys which both need to be plugged in before the Very Important Secret stored on them can be read. A very neat concept!
GEOM_SHSEC is very easy to use. As a root, simply do a:
gshsec label -v secret /dev/ad0 /dev/da1
newfs -U /dev/shsec/secret
mount /dev/shsec/secret /mnt
... and that's it.
Internally, SHSEC will generate a RC4 stream which is keyed to the system's sources of entropy (i.e. the key is presumably totally random), using the arc4random() call, and XOR it with the data written to the device in such a way that all components to which it is written must be present to reconstruct the original data.