Select the part you want
to connect to.

The first part moves to connect
to the second part, such that the ports are aligned and clocked.
The parts are connected at the selected ports. An engineering
connection is created between the two parts. For more information
about engineering connections, refer to Mechanical : Assembly
Design User's Guide : Engineering Connections.
The first part moves to make
the connection. If it is connected to a pipe, the pipe moves
to connect to it, after you update. In the image below, the
valve moves to connect to the adaptor.

However, a part already connected
to another part cannot be selected as the first part. Therefore, to connect to such
a part, the other part needs to be selected first.