What is the rising blood alcohol defense?

It follows directly from the absorption curve. If the State's evidence is a reading taken ninety minutes after driving, and the person was still absorbing during that window, the reading may overstate where they actually were behind the wheel.

What makes it work isn't the theory, which is uncontroversial. It's the facts. A meaningful delay between driving and testing. A reading close to the threshold rather than well above it. Evidence about when drinking stopped. And usually an expert who can put the numbers in front of a jury.

What defeats it is equally predictable. A long gap between the last drink and the driving, a reading far above the limit, or two samples showing a declining rather than rising trend.

Note the two breath samples are informative here. The relationship between them says something about where the person was on the curve.