Tree bast (fibrous material from the phloem) was used as cord for tying various things from roofing to kraals to even prisoners of war. This bast is extracted from the inside of fresh tree bark. It is then kept in water so that it remains soft enough for use because once used and exposed, the bast dries up and stiffen. It is very tough and can be extracted in different widths and lengths depending on the need. As such it is also used for measurement purposes. It is from this bast that our elders exclaimed that it would be similar to avoiding the real issue if one used the bast to measure the length of the snake when the snake is there for all to see.
The proverb is used to summon someone to address an issue directly and not shamelessly avoid the obvious. This proverb is particularly useful when there is enough evidence in a hearing and the one providing evidence is being indirect or outrightly avoiding the facts.