Liuxin Newman (Thimblelady), in order to keep slippery threads from getting unthreaded from the needle while sewing, secures the thread in a way that not only keeps the needle from unthreading, but also allows you to keep a longer length of thread on the needle that you release as you sew. After threading the needle, keeping the thread half on one side of the needle's eye and the other half on the other, take the tip of the needle and pierce through one thread close to the tail, then do it one more time further down on the same thread (it will look like the pierced thread forms an "s" shape; pull the needle all the way through the two places in the thread that you pierced and you will see that your two sides of the thread are now intertwined. As you tug on one end of the thread, you can lengthen the amount of thread available and the thread cannot come off the needle.
My explanation may sound confusing--it's the best I can do...
but if you have access to Episode 310, Liuxin Newman demonstrates this method. It is actually a very common way of keeping the thread from slipping off when embroidering, expecially with silk thread. I know my husband's grandmother in China used this method and my mother from Italy does the same. Maybe someone else can pipe up and explain it better than I if you don't have access to that particular TQS episode. I tried looking for a youtube video but could not find it.