How it works:
Step 1) Lower the Mini without landing, so the cargo cup touches the ground.
Step 2) Go up, the hook will gravity release and the cargo cup will flip upside down, releasing the payload!  No electronics, this is a 100% mechanical process.
Note that Mini can handle a payload of up to about 200G, however to prevent premature wear on the motors, limit payload weight to 80g.

Engineering Challenges:
-  Fishing line is super lightweight and strong, but it can get tangled in the propellers if tied directly to the drone. The straw harness provides the rigid clearence needed, to keep the fishing line away from the propellers.
-  When the drone goes forward, wind resistance from the cargo cup swings the harness back and into view of the obstacle avoidance sensors.  By using 2 straws in a V shape, the harness rests on the sides of the sensors, instead of on top of them, in a single straw design.
- In my original design, the cargo cup had a solid plastic bottom, which created a lot of wind resistance, slowing the mini down and wasting battery.  The fishnet sock bottom, allows air to pass through the cargo cup, reducing wind resistance, for a faster, more efficient flight.

Important: By using a payload carrier or adding a payload, you risk damaging your Mini and you may void your warrenty. I do not recommend it. It's a risk I'm willing to take.

Fun idea.  

Very imaginative

Interesting concept, Thanks’ for showing us it!   

Interesting

An advice from Dji to be careful and avoid unauthorized additions between 3,2,1 seconds...  

Smart thinking... nice one!

Hello there trader6777. Thank you for reaching out and for sharing this rig that you have created for your DJI Mavic Mini. Just a reminder that if accessories are mounted to the DJI Mavic Mini, it is recommended to enable the Payload Mode for enhanced safety. Please note that the Maximum Service Ceiling above Sea Level is 1500m and the maximum flight speed is limited when the Payload Mode is enabled. In addition DJI does not recommend using a 3rd party accessory when flying the DJI Mavic Mini for compatibility purposes. Finally, thank you for giving out your disclaimer with regards to using this carrier. Have a safe and a happy flying always.

Nice DJI!
I mean DIY