I noticed in my game (log attached) that I was able to briefly see an animation that showed which creatures my opponent had selected to sacrifice to Liliana, Dreadhorde General's -4 ability before I selected mine. My understanding of the rules is that both players must select their choices simultaneously before the other's is known so as to not gain any additional gameplay advantages that may affect their choices. I'm not sure why the animation was there but it was a brief trail of light going from liliana to the 2 creatures my opponent selected.