The Match Throw Arrow command is used to copy the throw arrow configuration on an existing diffuser to one or more other diffusers.
To match throw arrows, go to
Ribbon: DM HVAC->Diffusers-> 
Match Throw Arrow
Pulldown Menu: DM HVAC->Diffusers->Match Throw Arrow
You will be prompted to select the diffuser with the throw arrow configuration to be copied.
Select diffuser to use as throw arrow source:
You will then be prompted to select one or more diffusers to which the throw arrow configuration will be copied.
Select objects:
Press ENTER to change the throw arrow configuration of the selected diffusers.