What’s the difference between 61-8-401 and 61-8-406?

The primary difference is in a presumption -- which you have a right to challenge -- as raised by your BAC.  If you are over .08%, you are presumed to be impaired.  While you can attempt to disprove this presumption research by the National Highway Traffic and Safety Administration (NHTSA) has established that this level of alcohol affects your divided attention ability, your reaction times, and your level of control behind the wheel.  

There are minor differences in the penalties for these two offenses as well.