In addition to the OS's CSPRNG (either
/dev/urandom or
CryptGenRandom),
it also mixes in:
timestamps, down to the microsecond, of every keypress and mouseclick made during the wallet creation wizard. Also logs mouse positions on every press, though it will be constant while typing. ... Then we throw in the [name,time,size] triplets of some volatile system directories, and the hash of a file in that directory that is expected to have timestamps and system-dependent parameters. Finally, take a desktop screenshot...
Please note that it only uses the sources above if you're creating a wallet via the GUI. If you're using a simple script (e.g. the one I posted a couple of days ago for creating a wallet from a deck of cards), it only uses the OS's CSPRNG for additional entropy.
FYI Bither is the only other wallet I know of which goes to similar lengths for collecting additional entropy like this.