The first time I noticed someone receive a serve and then run up and spike it over the net I thought "Wait a minute -- don't back row people have to hit from behind the 3m line?" So I googled it.
What I mean by fake lineups is that the person designated to be in pos4 doesn't actually have to be in pos4, they just have to be to the left of the person designated to be in pos3 and in front of the person designated to be in pos5. This makes sense to me now but it was weird when it was new info. Same with an OP whose designated rotation spot is pos4 but then as soon as the serve happens they run over to pos2. Again, makes sense now.
Re: the libero. Lets say a team doesn't do a libero sub/switch, she just stays in and plays the whole game. Japan used to play this way with Arisa. Lets say she starts in pos6. After the first side out she "moves" to pos5. What happens after the next side out? She can't rotate to the front row. So does the player who was in pos6 just jump over her and into pos4? And does the libero remain frozen in pos5 forever, and has to always stay behind pos4 player and to the left of pos6 player.
I'm sure you guys and girls can, but I can't, look at a team waiting to receive serve and determine quickly who's really where they are supposed to be. I suppose I could, like counting cards in blackjack, keep track of everyone but that's too much brain work
You can search keywords like "volleyball rotation 5-1" to learn the relationship of rotation and position. (5-1 means 5 attackers and 1 setter, most of the top teams use this line up)
To know the rotation of receving team, I usually watch the setter's position first, then others will be easy to recognize.