Answer
Directional testing chooses the direction of a test based on the assertion and the risk of over- or understatement. Assets and income are usually tested for overstatement (vouch from records to support—existence/occurrence); liabilities and expenses for understatement (trace from source to records, and search for unrecorded items—completeness). It targets the more likely error direction efficiently.
Real-world example The auditor searches for unrecorded liabilities (understatement risk) while vouching assets for overstatement.