Why does the law cause people to lose control of their old sinful nature?
The law, the Word of God, comes as a cattle prod, exposing and provoking the old sin nature. Because that nature is the enemy of God, it cannot stay trained, inhibited, or dressed up; when confronted by the law, it reacts, and people suddenly lose control of it.
Les explains that a person can train, inhibit, dress up, and make the old nature look pretty good, but when the law—the Word of God—comes along, it acts like a cattle prod. Then suddenly people lose control of their old sin nature, because it is the enemy of God. The surrounding context connects this to Adam: by one man sin—the old Adamic nature—entered the world, and death came with it, as the curse fell on the whole creation.