If you’re sharing an always-on DSL or cable modem connection, a client computer can enable or disable it. If you’re sharing a dial-up connection, a client computer can connect or disconnect it, causing the ICS server computer to dial in or hang up. This can be a great convenience, since you don’t have to go to the server computer to take these actions. But it can cause trouble if someone disables or disconnects the server’s connection while other people are using the Internet.
See also “XP ICS – Internet Gateway” on PracticallyNetworked.com.