This option is extremely useful for post route optimizations for 7 Series designs. I recently had a design where route_design ended with 980 ps of negative setup slack and phys_opt_design -directive AggressiveExplore was able to reduce it to 69 ps of negative slack. At that point I moved one cell which was at the worst case path and another sequence of route_design -tns_cleanup + phys_opt_design -directive AggressiveExplore managed to push it to 15 ps of negative slack. At which point I stopped optimizing.