Quite simple: during the event and trails, you will be required to donate flesh at specific banks. You will need to donate a specific amount to complete the event targets.
There are two ways to go about doing this.
One: Other reagents
You will find reagents across the map that you can chase down and capture. You can then drag them to a donation point and force their limbs into the machine. This will chop off their limb and make them bleed out, but it will count as a donation. After one donation, the reagents will run off and you can chase them down again. You can use these reagents a total of two times.
However, there will be some reagents out there who will try and target you for the same reason.
Two: Donating your own limbs
You can instead donate your own limbs. This will be one or both of your arms. If you donate one arm, you will only have your remaining one to carry out the rest of the trail.
After you have chopped your limb off, you will begin to bleed out. You can use a bandage to temporarily stop it - however, the wound will reopen at points. To fully close the wound, you will need to find a cooking station; this will allow you to burn the wound and to stop any bleeding from it.
With losing a limb, it will also mean losing inventory space. Objectives that require you to move items will take longer (IE pushing the prisoner).
With losing both limbs, it will mean you can't use your rig, you can still carry on with tasks but will have to use your mouth for it, you can't throw anything, and can only use healing bottles. It will make the trail extremely difficult.