The circuit relies on the idle state of UART being a logic high signal. When the Tx pin is idle, the transistor stays on. This allows the Tx/Rx pin to pull Rx low when the AVR sends a 0.

For most people, they’ll be very happy to use the standard Nordic UART RX/TX connection profile. In this profile, the Bluefruit acts as a data pipe, that can ‘transparently’ transmit back ...