This command effectively does a /REDIRECT 0 WAIT. In other words, it suppresses all output to all of your windows from your current server for one full “WAIT” cycle (the time it takes for a command to go out to the server and come back)
This discards any incoming data from the server until a full second passes with no new data from the server. This also discards the contents of the window's “hold buffer” when you are in HOLD_MODE.
The FLUSH command is non-blocking, and does not interfere with the normal operation of other servers.
The FLUSH command cannot actually tell the server to stop sending you data; if you are being flooded, you will simply have to wait until the flood stops.