OSRM partition generates data for MLD(multiple-level-dijkstra), and its an implementation of CRP.
Partition is the most important step in CRP which would influence custmization speed and query performance dramatically. OSRM's Partition implement inertial flow, which tries to bi-set graph based on dinic.
Here is a simple graph to walk you though OSRM partition's logic, for more details please go to detail page.