It is a harness loop or line.
Windsurfer use it on their boom to hook into the harness so they don’t need to hold the boom with just their arms…so their body weight will hold it.
We used this also in the beginning of kitesurfing, for the same reason. First bars didn’t have chicken loops, so you had to hold the bar with just your arms…very tiring.
When the Chickenloop and 4 lines came, we used this loop to have the kite powered up when jumping. When sending the kite for a jump, you hooked into this loop for full power.
Then the Microloop was developed, which has the same effect, but much smaller. I still use the Microloop for my style…check out this video: https://strictlyhooked.com/viewtopic.php?t=2465
Or use the search on kiteforum for Microloop.