This parameter allows you to ignore all subsequent alerts/commands until you turn it back off. Every command will still be seen, however they will be output only as debug in your log and not forwarded to the exchange.
Typical usage is pyramiding. If you get a long alert, you can include testing=1 after your open command. It will then ignore all your other long alerts until you get a short alert with testing=0 before the short command.
Any alert or command seen between testing=1 and testing=0 will have d=1 added to it.