How does Stitching capacitor provide shortest return current path between plane splits, as we know that it blocks DC current?

I heard that Stitching capacitors are used to provide shortest return path whenever a signal is routed along the 2 power plane splits. As we know the return current path will be right under the trace. So how does the stitching capacitor provide shortest return path? As we already know that capacitor blocks DC current. So when I connect a stitching capacitor, how does return current flows through the capacitor?