Spit splices are wonderful. They produce a strong join that's virtually indistinguishable from the rest of the yarn when knitted up and there are no ends to weave in after. Weaving in ends is the bane of my existence so spit splices are a Godsend. They work on most natural, non-superwash fibers. This is Classic Elite Wool Bam Boo, which is 50/50 merino and bam boo.
You start with two ends of the same yarn. This doesn't work when you're changing colors, by the way. In that case, you need to do a Russian join.
You strip off half the plies from about 3" of each end so the joined yarn won't be bulkier than the rest of the yarn. This is a 5 ply so I removed three plies from one end and two from the other.
You dampen your hands or the fiber with spit using whatever method you wish. You can spit on your hands, lick them or just stick the ends of the yarn in mouth and soak the fiber that way. However you do it, the yarn needs to be saturated or the ends won't fuse. You can't cheat and use water. The yarn Gods will not be fooled; they demand saliva. You line the two ends up on the palm of your hand.
Then you rub your hands together like crazy. The heat from the friction is what fuses the two ends together so this is no time to be gentle.
When you're all done, it looks like this. The join is strong enough to withstand tugging. It won't come apart while you knitting it even when you're knitting a gansey, which requires that you maintain a constant, even tension on the yarn.