For back walkovers I typically teach it using the same hand placement as a handstand because esentially is a handstand step down, just the lead into it is different. I've also taught it using a different "backhandspring hands" than CoachLin uses---for this one, one hand is diagonal (turned slightly outward so fingers can grip the beam) and the other hand is placed in front of it facing forwards---this way your front hand/area by wrist can fit into the L shape (made by thum and index finger) of your other hand. Or you could always just put one hand slightly in front of the other.
All in all, you should do what ever hand placement feels most comfortable to you. Some gymnasts lke putting their hands like handstands (but I wouldn't reccommend this for backhandsprings), and some put them as CoachLin described, and some put them as I've described or other ways. Long story short, put them how you feel most comfortable!