Play when someone else discards a Rune. Pick up that Rune for your own
Play immediately after another Rune is played. Copy that Rune as if you had played it
Let's say we're talking about the Lightbringer Rune as being what was originally played. Here's my understanding of what happens:
1) Lightbringer is played (owner gains 1 Power and 1 VP)
2) Chaosium is played, and they pick up Lightbringer
3) Illusion is played, meaning Chaosium has effectively been played again, meaning the last Rune that was discarded (Chaosium) is picked up by whoever played Illusion.