Withholding is how employers prepay income tax on behalf of employees during the year. The W-4 form tells the employer how much to withhold. Many workers misunderstand the purpose of the W-4: it’s not about lowering taxes permanently, it’s about matching how much is withheld to estimated tax liability so you neither owe a big balance nor give the government an interest-free loan via a large refund. Small life changes (new job, second job, marriage, child) often require a W-4 update. This question asks which action the W-4 specifically controls and which it does not, so you can manage payroll withholding intentionally.