Right-click the Item by the player, which is supposed to receive damage. The linked tether item needs to be inside of the inventory of the player who should be protected. If the protecting player has a tether item in the inventory, damage will not be transferred.
Shapeless crafting from 1x string. Found in the creative tab Tools & Utilities.​
If you want the source code, just send me a message.
Feel free to use in modpacks.