An Application Kill Switch is a feature in MacSentry that allows you to define applications to be automatically killed when you disconnect from the VPN. It is useful to protect your real IP from being exposed if you are using a program and don't want your real IP to be seen if you get disconnected from the VPN.
Say for example you are downloading from BitTorrent and don't want your IP to visible. While you are connected to the VPN your IP is hidden, if you get disconnected for whatever reason your BitTorrent will try to automatically reconnect you with your direct internet connection exposing your IP. With the Kill Switch enabled you can set it up to kill your BitTorrent client if you get disconnected. This will close your BitTorrent client before it reconnects and ensures your real IP is not exposed.