To be able to determine whether it is more likely that the driver was absorbing or eliminating alcohol when stopped, the first fact that need to be known is when the person stopped drinking. Without that fact, any assumption concerning the person's status as absorbing or eliminating alcohol at the time is nothing but speculation. The curve defense attempts to create the curve that establishes the alcohol concentration was lower at driving time than at testing time by presenting evidence of the time of last consumption, the rate and duration of consumption, and from those facts, constructing a predicted alcohol concentration absorption and elimination chart. The state-created charts thumb nailed on this page are often used to assist in creating this curve. The law prohibits being intoxicated at the time that you're driving, not some later time when you're finally tested.
Alcohol is absorbed and eliminated from the body over time.
A test result obtained an hour or more after you were stopped doesn't state your alcohol concentration at the time you were driving. This seems obvious, and it is. The underlying assumption is that absorption of alcohol into the blood stream is very rapid. The truth is that it isn't, and that complete absorption can take as long as three hours, even when the subject had an empty stomach during drinking. That legal fiction can be challenged, effectively.