How does preventing grace lighten the effects of original sin?
It prevents the full consequences of our alienation from God and awakens conscience, giving an initial sense of God and the first inclinations toward life in its fullest sense.
Catechism for the Global Methodist Church, question 51
45Then he opened their minds to understand the Scriptures.
37All that the Father gives me will come to me, and the one who comes to me I will never cast out.
32And I, when I am lifted up from the earth, will draw all to myself."
14A woman named Lydia, a dealer in purple from the city of Thyatira, who worshiped God, was listening; the Lord opened her heart to pay attention to what was spoken by Paul.
7Because the purpose of the flesh is hostility toward God, for it does not submit to the law of God, nor indeed is it able. 8Those who are in the flesh cannot please God.
14The natural person does not receive the things of the Spirit of God, for they are foolishness to him, and he is not able to understand them because they are spiritually discerned.
Preventing grace is God's first work in a fallen person, and this question asks what it accomplishes. ¶102.3 of the Book of Doctrines and Discipline says that preventing grace "prevents the full consequences of humanity's alienation from God and awakens conscience, instills a basic knowledge of the moral law, gives an initial sense of God, and restores a measure of liberty to receive the further graces of God, all of this issuing in the first inclinations toward life." Three things happen in a person before he ever believes: the worst effects of sin are held back, conscience wakes up, and a first sense of God stirs together with a first inclination toward life.
Without that grace the human condition is grim. Paul says "the purpose of the flesh is hostility toward God, for it does not submit to the law of God, nor indeed is it able" (Romans 8:7), and "the natural person does not receive the things of the Spirit of God, for they are foolishness to him" (1 Corinthians 2:14). Article VII of the Confession of Faith says the same: fallen humanity, apart from grace, "is destitute of holiness and inclined to evil." Left alone, the human will does not want God. Those are "the full consequences" of alienation, and every person would live under them if God did not intervene.
God does intervene, and he does it from the outside. The risen Jesus "opened their minds to understand the Scriptures" (Luke 24:45). At Philippi, Lydia was listening, and "the Lord opened her heart to pay attention to what was spoken by Paul" (Acts 16:14). Jesus promised that when he was lifted up he "will draw all to myself" (John 12:32), and that "the one who comes to me I will never cast out" (John 6:37). The drawing is his work; the coming is the response his drawing makes possible. An awakened conscience and "the first inclinations toward life in its fullest sense" are that drawing felt from the inside.
So the church expects to find conscience, moral seriousness, and a hunger for God in people who have not yet believed, and it receives those things as signs that God is already at work. God opened Lydia's heart while Paul was still talking, and he may open another heart during any sermon. The preacher's task is to speak the word that grace uses (Acts 16:14).