You get 8 or 10 big money movers to agree to collectively "pump" a market, what's to stop one from "dumping" early, screwing the rest of you?
I have considered this as well. Think of it as a club where rich guys all gather in a smoke filled back room.
You all agree to do something, you all do it then show proof that you did it.
Those that do not do it are no longer get to join those rich guys in the smoke filled back room to find out when the next move is.
And the key to being part of it is to make money. As long as there is long term incentive to keep making that money then there is less incentive to try to screw over the group.