Can anyone tell me what this piece is for?

For connecting your kite-leash in handle-pass mode, one D-ring around the line on the back of your harness, the other to the leash.

also works great if you want to hook in suicide mode + safety @ your chickenloop.

1) D-ring onto your leash.
2) leash to your safety @ CL
3) Around your CL and other Dring back in your leash.

This way you ride suicide mode for unhooking but if you fire your safety it will still depower as usual.

(North 2011 is the only bar I know which has this as a standard feature. Get to work Rudo!)