When you consume alcohol, it takes time for it to absorb into your system. Essentially, you could have a number of beers or alcohol and not feel the effects to sometime later after you stop drinking. When a breath test is given you at the police station, it could very well be up to an hour after you were last driving. The breath test will give a result for what your BAC at the time of the test, but what about your BAC an hour prior, when you were actually driving? Retrograde extrapolation is the formula for determining BAC at a prior time. This may very well lead to your BAC being below the legal limit at the time you were actually driving, even though it was above the limit at the time of the test.