What does nprobe do, and how would you pick it?

Nearest-centroid assignment partitions vector space into Voronoi cells. A query’s true neighbor often resides across a bisector, and nprobe dictates how many of these cells are searched. The computational cost is defined as nlist + nprobe * (N/nlist). Because nprobe behaves differently depending on the corpus structure, it must be sourced from empirical recall@k curves rather than relying on general heuristics.