Well, I’m a beginner but I’m learning. Like you say, the fibers are too short and must be continually spliced together throughout the length of the string.
First off, when you being twisting the fibers into reverse twist cordage, you start with one tag end longer than the other so they don’t need splicing at the same time (a no-no….much like laying short hardwood floor pieces next to each other). As you get within a few inches of the tag end, you gather another bundle of fibers and prepare to splice them in. You can either overlap the new bundle and the single tag end by a few inches and continue twisting, or form a “u” with the new fibers being twisted into both strands of the cordage….thus being stronger and less prone to pull out. On a bowstring, I think it’s best to use the “U” method.
None of this makes much (if any) sense in writing, but seeing it in person it’s pretty easy. I find the hardest thing to do is to keep a consistent string diameter while continually splicing in additional fibers.
