The Single 45 Connection command inserts a diffuser connection that extends at a 45-degree angle to the selected duct, then turns 45 degrees to connect to the selected diffuser.
To insert a single 45 connection, go to
Ribbon: DM HVAC->Diffuser Connections-> 
Single 45 Connection
Pulldown Menu: DM HVAC->Diffuser Connections->Single 45 Connection
You will be prompted to specify the diffuser to be connected.
Select diffuser to connect:
You will then be prompted to specify the duct to which the diffuser will be connected.
Select duct or label at connection point:
Select the duct at the location where the connection will be made. The duct will be broken at the selected point. A duct that is 45 degrees to the selected duct will be inserted and connected to the selected diffuser.