This method of adding someone to a conference temporarily put your connection to the conference on hold. That allows you to hear the call progress and talk to the whoever answers the new call without the existing conference parties hearing that conversation.
The other parties already in the conference are able to continue talking to each other whilst you try to add the new party. You can also add a new party without using hold.
If they answer and want to join the call, press Conf again. This call and your current held calls are turned into a conference call.
If they do not want to join the call or do not answer, press Drop. Then press the appearance key of the previously held call to reconnect to that call.