Put -Dpaper.explicit-flush=true in quotes

src/main/java/ac/grim/grimac/manager/init/start/TickEndEvent.java

@@ -3,7 +3,10 @@ package ac.grim.grimac.manager.init.start;
 import ac.grim.grimac.GrimAPI;
 import ac.grim.grimac.manager.init.Initable;
 import ac.grim.grimac.player.GrimPlayer;
+import ac.grim.grimac.utils.anticheat.LogUtil;
 import ac.grim.grimac.utils.lists.HookedListWrapper;
+import com.github.retrooper.packetevents.PacketEvents;
+import com.github.retrooper.packetevents.manager.server.ServerVersion;
 import com.github.retrooper.packetevents.util.reflection.Reflection;
 import io.github.retrooper.packetevents.util.SpigotReflectionUtil;
 import sun.misc.Unsafe;
@@ -30,6 +33,10 @@ public class TickEndEvent implements Initable {
             return;
         }
 
+        if (PacketEvents.getAPI().getServerManager().getVersion().isNewerThan(ServerVersion.V_1_11_2) && !Boolean.getBoolean("paper.explicit-flush")) {
+            LogUtil.warn("Reach.enable-post-packet=true but paper.explicit-flush=false, add \"-Dpaper.explicit-flush=true\" to your server's startup flags for fully functional extra reach accuracy.");
+        }
+
         // Inject so we can add the final transaction pre-flush event
         try {
             Object connection = SpigotReflectionUtil.getMinecraftServerConnectionInstance();