Among the injuries the NHTSA reports from improper placement of the hands when an airbag deploys are amputations of fingers or entire hands, traumatic fractures and a particularly stomach-churning injury called "degloving," which — trust us — you definitely don't want to look up.
Degloving is nasty. That by itself should be enough to convince you that you should keep your hand/arm away from the path of the airbag.