Parachutes properly deploy 99.9% of the time. Even when skydiving equipment is frequently assessed and replaced, the unexpected can happen. Even seemingly properly packed parachutes can fail, with one in every 1,000 skydives not always operating at 100% efficiency.
So when a skydiver finds himself in a position where his main parachute isn’t opening or has opened with an error, they simply remove that parachute and deploy their reserve parachute instead. This is called „cut-away procedure”. Known as „Fuck you – Rescue you” rule ;).
If the skydiver is unable to deploy the main parachute or becomes unconscious at any point during the skydive, the AAD comes into play. It will automatically deploy the reserve parachute for you, allowing you still to land safely.