The Transparent Connection command inserts a connection between a duct and a diffuser for which the graphics will not be drawn. Airflow will be transferred from the diffuser to the duct.
To insert a transparent connection, go to
Ribbon: DM HVAC->Diffuser Connections-> 
Transparent Connection
Pulldown Menu: DM HVAC->Diffuser Connections->Transparent 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 to connect:
A duct centerline will be inserted between the selected duct and diffuser. The Graphics for the centerline will be set to Transparent.