Body Mounted Attitude Gyro
Simply put, Body Mounted Attitude Gyros (BMAGs) are rate gyros mounted in a fixed reference to the spacecraft itself.
As a backup to the complex IMU, the Apollo CSM engineers decided to use rate gyros, much like those in the IMU, and mount them to the spacecraft directly. These gyros can then give the rate of angular change referenced to the spacecraft instead of inertial space.
Maintaining attitude in space requires two separate functions. Remembering a reference attitude and knowing the rate at which the spacecraft is turning. By adding the angular rate to the remembered reference, the current spacecraft attitude can be calculated.
The BMAGs determine the angular rates and the Gyro Display Coupler (GDC) receives these angular rates and adds them to the current attitude, thus remembering the attitude difference from the original reference in real time.
This was a primitive, but reliable backup to the CMC for attitude reference, and allowed Apollo 12 to monitor the ascent when lightning caused the CMC to loose it's inertial reference during the boost phase of flight.
