In William Faulkner's story "Barn Burning," Abner has anger issues. He sets barns on fire in a retaliatory way to take out his anger and frustration on people who have more power than he has. The notion of power, or lack of power, is really what drives this character. Abner must assert himself through violence--either whipping a mule or setting fires or hitting his son--to make himself feel in control. Thus, when he whips the mule without heat and in a pre-emptive manner, he shows his desire to assert control.
Likewise, descendants of his will over-run the motor in a pre-emptive attempt at control or power. Faulkner's commentary, then, on the effect one person has on the next, or ancestral impact, is that the sins of the father are the sins of the son. The way one person behaves (or misbehaves) is picked up and mimicked the by next generation. In this story, Sarty could potentially follow in his father's footsteps and become a violent, impotent man who sets fires, yet he chooses otherwise at the end of the story. Therefore, Sarty will not take the sins of his father on--he is free and will be a different person.