The safe reading is in play due to dji setting it up that way. Apparently if it reaches the set amount it must have an ability to cause permanent damage to the aircraft. The red is going to only appear when the voltage reading reaches in the 3.4 range. There is likely two possibilities you will see this happen. It would be either from remaining charge, or one of the battery cell's is bad not allowing it to maintain the supplied power. Full throttle will give the fastest way to reach the red. Flying into a headwind and trying to maintain a higher speed, or full throttle while climbing in altitude are both drain power the most.
If you see red, or are close to reaching it, all you have to do is back off the lever and not ask for as much power.