Root wrenching loosens the soil and aerates the roots which helps produce a seedling with a more fibrous root system and usually a greater shoot-to-root ratio.
Root wrenching is a modified form of undercutting. Specifically, the seedling taproot is periodically undercut at different depths during the growing season. Either a fixed or reciprocating tilted blade is used. The objectives of root wrenching are to hasten seedling dormancy and develop a short fibrous root mass.