The propulsion throttle is something new DJI added in the 1.6 firmware. In order to prevent the battery from reaching a dangerously low voltage, the propulsion will be limited and/or your Inspire will auto land when the battery reaches the critical level. Both are fail safes that aim to prevent the battery from reaching 3.0V (where it would turn off mid-flight).
It's generally a good idea to land before the first battery cell reaches 3.3V. As you can see in your screenshot, you're battery is right at about the level where you should be considering landing.