"sheep dipped"

The phrase "sheep dipped", while it may not be well known in the regular military, is commonly used in intelligence circles. A way of saying someone has been given an alternate identity. The best known example being Air America, but also in many other covert ops applications. Not necessarily military. "Those who were accepted got "sheep-dipped" and vanished."

Do you know why (i.e. the relationship between literal sheep dipping and vanishing)? Pamela