I used one long ago, they work OK if the woods are clean. They had a snap off plastic fitting to put on your arrow that had a straightened fish hook in it that would imbed in the deer if your arrow passed through and broke off, the line was attach to the fish hook.
I shot one deer with a tracker string, I was shooting wheels and sights and found no trajectory difference to 20 yard but the arrow slowed down quickly after that. On the deer I shot the string made it most of the way to the deer but broke off on a bush about 15 yards from the dead deer.
I wouldn't use one today, my tracking skills have improved over the years and I seldom loose a deer now.