The Bottom Connection command inserts a diffuser connection between a duct and a diffuser directly below it.
To connect a diffuser to the bottom of a duct, go to
Ribbon: DM HVAC->Diffuser Connections-> 
Bottom Connection
Pulldown Menu: DM HVAC->Diffuser Connections->Bottom 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 connection between the duct and the diffuser will be inserted.