[foxtrottbravo]

In a normal NATed setup your ports are closed from the outside until a client from inside your network Starts sending packets to the outside.

The Router will keep track of network package going through to the Internet and store it in a table. in case there is an answer from outside of your network the router will look up whether a client started this conversation (there is a corresponding entry in his table) and will forward the incoming packet to the client that started the conversation.

What nintendo is asking you to allow here is to allow any outside packet coming in over UDP to get to the switch whether it first asked for it or not.

This means in practice you won't be able to run any other service which needs an UDP port fowarded in your network. It also means anyone can talk directly to your switch on any port they like whether you want to or not.

And it means, that should something ever take/get the same IP as the switch it will be exposed to the Internet directly