The idiom beautifully combines two parallel metaphors. `追根 (zhuī gēn)` is “chasing the root,” an agricultural metaphor for digging down to find the very foundation of a plant. `溯源 (sù yuán)` is “tracing the source,” a geographical metaphor for traveling up a river to find its starting point. Together, they create a powerful image of a comprehensive investigation that goes both deep (to the root) and back in time (to the source).