Attrition Through Enforcement means that as existing immigration laws are enforced, more and more illegal aliens will find it difficult to work in the United States. Likewise, enforcement of existing immigration laws will make the United States less enticing for illegal aliens to sneak into in order to earn higher wages.
It is not necessary to spend money and law enforcement effort to round up the millions of illegal aliens who have taken jobs from Americans. By enforcing our existing laws, illegal aliens will self-deport - meaning that they will pay their own way to voluntarily go back home and reunite with their families.