I don't know if this is an existing tool that everyone knows about but I came up with this little hook thing when I was seizing the tail of the futtock shrouds to the main shrouds. You just reach in around the shroud and pull the small hook back toward you, lay the thread in the small hook, push it back through, rotate it 90 degrees and pull it back out. If you rotate it the right direction the thread drops out of the small hook into the big hook and you can just pull the loop through. If you pinch all the threads as you are pulling it back you can keep tension on the thread until you make the next loop. It was taking me 10-15 minutes to do one shroud and now it takes about 1 min. Unfortunately I was almost done when I figured this out. Hopefully I can find more uses for this. It sure made a fiddly job easier. It doesn't show that well in the picture but the hooks are 90 degrees to one another. Hope you understand my explanation. 
