I thought they could only challenge if there is a stoppage ie. a whistle is blown
Yeah that's my understanding.
So that means it can only be used to argue referee decisions leading to the stoppage, or non decisions on the same tackle leading up to the stoppage.
Any other tackle where a non decision that should have been a decision can't be challenged , for example in our trial on the weekend where replays showed a Penrith player very clearly knocked on next to the sideline on the first or second tackle, which was missed, and a try was scored later that set. There's no stoppage, so there could be no challenge.
I don't mind the idea, but it's pretty obvious it doesn't address half of the problem it's trying to solve, which is the refs not blowing the whistle when they should have.