A Funeral Treatment is when you're driving and someone is tailgating you and you slow down to very low speed, causing a line of traffic to build behind you. This makes it look very similar to a Funeral Procession. It doesn't have to be tailgating, you can give someone the funeral treatment for any reason.
Joe: Damn, this guy is riding my ass, I keep giving him a break check and he still won't back up.
Mike: Give him a funeral treatment, then he'll back off.
Joe: Speed limit is 30, I'll give him a 10mph treatment, that should piss him off.