This defense is based on the idea that alcohol levels change over time, as the body absorbs alcohol, reaches a peak level, and then eliminates alcohol. Breath or blood testing is done after driving (sometimes long after); these test results tell us what the alcohol level is at the time of testing, not at the time of driving. The rising alcohol defense is simply that at the time of driving (the critical time in a drunk driving case), the alcohol level was below the legal limit, even if it continued to rise until the time of testing.