The parachute activates when encountering a free fall or extreme pitch and role angles(assuming loss of one blade due to prop breaking, falling off, or loss of power to motor). Better to loose props than the drone. What this is for specifically is to get waiver to fly over groups of people. Hopefully it will never get deployed, but if it does you’ll get drone back with very minor damage if at all.
ParaZero PZ-SA-2P Overview
Protect your drone and everyone under it with a SafeAir Drone Safety System for DJI Phantom 4 Series (not RTK) from ParaZero. The SafeAir is a parachute deployment system for your Phantom 4 series drone. It's powered by a built-in battery that is USB-chargeable and lasts approximately one hour before needing to be recharged. The SafeAir is an autonomous system that will deploy on its own when encountering either free fall or extreme pitch/roll angles. Once one of these scenarios is detected, a section of the SafeAir will twist to stop the propellers by inserting flexible stoppers into all four blades, at which point the parachute will be deployed to help bring your DJI Phantom safely to the ground.