Broadly, branches become less likely to interact as they become more dissimilar; dissimilarity tends to bring about dissimilarity, so they can quickly diverge to a point where the probability of further interaction is negligible. Note that at least as Eliezer tells it, branches aren't ontologically basic objects in MWI any more than chairs are; they're rough high-level abstractions we imagine when we think about MWI. If you want a less confused understanding than this, I don't have a better suggestion than actually reading the quantum physics sequence!