Apex Anvil question

I just got my vise today and love it but have a question. What is the sping for that is in back of the jaws for? it does not hold any thing.

If it is on top and directly behind the vise jaws, it is what is called a material clip or holder. As you tie in material on the fly, you can insert the material into the spring so that it will hold the material out of your way until you are ready to use it. For instance, lets assume you are tying a bugger. After you tie in the tail and you tie in the chenille for the body, you can insert the end fo the chenille into the spring so that you will have room to tie in the hackle. Hopefully, this is clearer that mud....