FLOOD_RATE can be set to the number of messages per second you wish to activate flooding. If messages from a user outpace FLOOD_RATE for FLOOD_AFTER number of messages, ON FLOOD is activated. If FLOOD_RATE is larger then FLOOD_AFTER, then you will end up seeing at least FLOOD_RATE messages before flood activation (If FLOOD_RATE is 5 and FLOOD_AFTER is 3 then you must receive at least 5 messages before the flood rate can be 5 messages per second).
See Also: on/flood set/flood_after set/flood_users