As far as I know (from experience as Tavlin, not from seeing the code), Zombie General gives the necro a chance, each round, for each zombie, to redirect to what the Necro is swinging at.

So if you're fighting the Massive Giant and a rager comes and attacks you and you redirect, your zombies don't all suddenly redirect at him (which is what I thought would happen when I first took the edge, IMAGINE MY SURPRISE heh).

However, each round there's a chance (on each zombie, I think) for it to redirect to the rager.

It's a highly useful edge during raid situations. It is so choice. If you have the means, I highly recommend picking one up.