Consumption After Driving is actually part of the Washington State Driving Under the Influence statute. Most DUI investigations begin the same way: law enforcement observes a moving vehicle commit a traffic violation, they stop the vehicle and eventually arrest the driver for DUI. The legal concept of consumption after driving recognizes there are occasions where law enforcement suspects someone drove, but they didn't observe them behind the wheel. There may be a gap in time between the driving and law enforcement contact and during that gap the driver may have consumed alcohol.
In Washington State consumption after driving is a defense to a DUI charge that must be proven at trial. If the judge or jury believes enough alcohol was consumed after the driving occurred to raise the driver's breath/blood alcohol content (BAC) to .08 or more, then the judge or jury should not convict the driver of DUI.
Consumption after driving is relatively rare, but the most common scenario is when law enforcement finds a single car collision and no one is at the scene. If the owner of the vehicle is found nearby and is intoxicated, then they will often be arrested for DUI. However, in this scenario law enforcement was not with the vehicle owner the entire time between the driving and being found, so there is a possibility she or he consumed enough alcohol after driving to have a BAC of .08 or higher.
There are other occasions where consumption after driving could arise. There are cases where individuals actually consume alcohol in the presence of law enforcement after driving. This is not smart since it could lead to additional charges, like Obstructing a Law Enforcement Officer and Open Container in Public, but it could make the defense of consumption after driving available once the DUI goes to court.