if you have a problem of your tying thread backing out of the tube on your bobbin holder and have to rethread it, you may want to try one of these ideas.

take a look at the first two items in the following link. the first one are miniature o rings that would slip over the tube of the bobbin and capturing the thread along side of the bobbin holder tube

the second items appear to be caps from a grazy glue bottle or something similar that would also capture the thread along side of the bobbin holder tube

http://www.lakelandflytying.com/43/Bobbin_Holders.aspx

bear in mind that these are just ideas that are you not required to do. just ideas