Mrs T as far as I'm aware 'ACTING IN CONCERT' (had to shout somewhere ;p) is just what it says on the tin: they are all acting together to achieve one goal, and should be convicted the same way as jointly responsible for the outcome (they all had the same mens rea), even if in fact only one of them carried out the actual physical act that killed him.
eg. Scenario 1. Bobby gets a gun to go kill Mikey. He arrives at Mikeys and shoots at Mikey, but misses. He is not charged with murder. Ok Mikey is not dead, but bear with me.
Scenario 2. Bobby gets a gun to go kill Mikey, Teddy comes along to help him, also with a gun. Bobby shoots at Mikey but misses. Teddy shoots at the same time, hits and kills Mikey. Bobby is charged with murder, even though he misses just in the same way as in scenario 1.
It is, it turns out, fairly easy to prove that Bobby went along to shoot Mikey dead, he does not get off just because he is a poor shot.
In the case of the heart-attack-victim in original post, judging by the appeal information it is known that one rock hit the man on the head, leading probably to heart attack, they have difficulty showing who, if any of the accused, threw it. Even if they could, how do you prove that the others, in trying to hit him (or the stumps) with stones, were trying to deliver a fatal blow? Throwing rocks at someone for fun is terrible terrible thuggery, but is it *manslaughter*? Because someone else beside you threw a rock that killed him? You have a dead man, and a bunch of rocks and stones. But the law (and this is a very very good law, I have lived in places which do not have it), requires you to clearly prove that those you are prosecuting for the crime were guilty of his death. I just don't think you can do that here. Do you?