The purpose of the whip was not to beat the mule team or to cause them to pull harder to avoid the pain of the whip. The crack of the whip was the critical part. The driver might lightly touch an animal with the whip, or pop the whip above their ears. He prided himself upon his delicate use of the whip. As has already been stated here the term "skinner" was not applied literally. A team of mules was far too costly to have a driver that raked their backs with a whip causing gashes and open sores that would get infected or draw flies.   Anyone who caused any harm or damage to a mule in the team would likely discover just how much damage a bull whip could do when applied directly to the flesh.