The host seems to be relaxed and happy, things just happen. You try and meet people at the door and introduce them to other people so they feel at home. A good host points out the bathroom and general layout of a party to newcomers. They make sure there is ice and that the music is creating the right vibe. If there's a fight, they call the police and meet them at the door.
Nobody sees the week of planning that took place before.
Undertake As in to assume responsibility. As host, you undertake responsibility for the gathering, whatever the outcome. However, the real power is in the people in your gathering, you are really working for them. A big part of being a leader is artfully also undertaking the artful negotiation of the slopes of perception. Leaders are the people who perceive what the group as a whole and see what the group is doing before anyone individual in the group notices.