Spit felting is a way to join two strands of yarn together with out a knot or with out ends hanging out. It only works with animal fibers, I repeat it only works with animal fibers. This is because animal fibers have scales on the individual fibers that allow them to lock together.
Things that will NOT spit felt are: Super wash anything, cotton, linen, silk, bamboo, rayon, pretty much anything that is a plant fiber. All of these fibers don't have scales. The reason it won't work in super wash is because that fiber has been treated and changed so that it's scales have been removed.
However Spit felting will work on a blended fiber as long as there is about 35%-50% animal fiber in the blend. For example I've never had a problem spit felting a 50/50 wool silk blend.
Now, how does one spit felt. Here is my example. I want to join the end of one ball of wool to the beginning of another ball of yarn. (I'm using two different colors of yarn so you can see the process more clearly.)
You are going to untwist the plies. If it is a singly ply you will just fray the end of the yarn and untwist it. Try to give yourself about an inch and a half to two inches of untwisted plies.
Now get those untwisted plies nice and wet with spit. Yup these puppies are soaked in my spit. You can either stick the yarn right in your mouth our find a more lady like way to do it, but lets be honest your going to get a little fiber in your mouth either way.
Intermingle the two ends making sure they get nice and tangled into each other. Now for the magic. Either rub the intermingled fibers between your hands or on your leg. Make sure to rub back and forth pretty vigorously.
Once your fibers are felted together they will look something like this, one glorious continuous length of yarn. You can continue to knit, with out worrying about knots or ends to weave in later,