Jumping jacks was an example I used because it is completely out there, but most people will find it an insane request and their brain will snap out of it. Generally becoming helpful at that point.
I've always given direct orders to anyone that I saw. Either by pointing at them, or by naming something they are wearing.
You're absolutely right about crowd mentality, too. You have to expect that most of the bystanders will be almost useless and order them about. I never told them to do jumping-jacks, but I do give specific instructions to specific people and that works a lot better.
But I know what you mean about seeing horrible things. I try not to think about Grandma lying in a pool of blood, even though I saved her. I hope I never see another compound fracture again.