User blog comment:KevlarNinja/Zande vs. Zulu/@comment-2136224-20101230225749/@comment-422690-20101231165809

That was cotton soked in saltwater, which is made to withstand arrows. The reason the armor stopped it is because it was tough and flexible, so the arrow would be more likely to bounce off. The cowhide, IIRC, is stretched tight. That increases the tension on the cowhide itself, and makes it easier to split.