The officer tasers the man in front of his pregnant wife and then threatens the wife when she gets out of the truck. The officer states that the man was tasered for not obeying him. We can see other such uses of the tasers as a corrective punishment for citizens not obeying officers, here, here, here, here, here, and here.
I fail to see the justification in the use of force here. The man could well sue for the tasering. While he was not responding immediately to the officer, he was primarily contesting that there was a speed sign as claimed by the officer. The entire disagreement was over his refusal to sign the ticket. The officer does not attempt to call back up or insist on the man signing the ticket before he can leave. Instead of continuing to speak to the man, he orders the man out of the truck, pulls out his taser, and shoots him. While the man turns back toward his truck, it should not result in such a sudden escalation of force.