Touching somebody while asking them to do something for you works in most situations, not just a traumatic one. It creates a heightened level of intimacy, which makes the other person feel more obligated to help.
I'd worry about karma!
Yep, the increased sense of intimacy that can make somebody feel more obligated to you can also make somebody feel uncomfortable and pressured. It's a technique that can definitely backfire. Using it with strangers won't necessarily give you the result you thought it would. However, I suspect in a "call 911" type of situation the other person is already uncomfortable and it's worth psychologically pulling them closer in order to focus their attention.