You arrive sober to a party at 11:00pm. You attempt to remain sober but decide to enjoy 3 shots of tequila at 1:00am. Right after the shots you decide to leave the party. As you leave your blood alcohol concentration (BAC) is rising. When you've driven 10 miles your BAC may still be below the legal limit (perhaps .04%). By the time you've driven 20 miles your BAC may now be over the legal limit (perhaps .09%). By the time you arrive home, 30 miles away, your BAC may be twice the legal limit (perhaps .16%).
If your body is still absorbing alcohol at the time the blood alcohol testing is administered...
the test results will not be an accurate reflection of your blood alcohol level AT THE TIME OF DRIVING. Remember, the only legally relevant time is the time you were observed driving.
Your BAC at the time of stop or police observation of driving may very well be BELOW the legal limit...
Your alcohol absorption does not stop just because your vehicle is stopped. By the time you reach the station and take your chemical test (blood, breath, or urine), usually 30 plus minutes after the initial stop, your BAC may have now RISEN ABOVE the legal limit.
The unfortunate reality is that California DUI law makes it extremely difficult to beat a DUI at trial. The law does this by allowing prosecutors to present to the jury the legal presumption that the result of your chemical test is presumed your BAC at the time of driving. Makes no sense. As a DUI defense attorney, I will try to establish by affirmative evidence that my client's BAC was BELOW the legal limit at the time of driving, so no weight is given to the results of the chemical test. The "rising blood alcohol" defense is strongest when your test results show a BAC slightly over the legal limit. Also, the defense works best where a credible drinking history can be presented (witness statements, receipts, etc).