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
It's not a firewall acl, it's a port forwarding rule. In a NAT, UDP connections you initiate will be added to NAT table temporarily to handle replies. Nintendo's port range covers all the ephemeral ports that OS will naturally give to UDP connections. So your computer may attempt to start a UDP communication but the reply traffic will be forwarded to your switch instead of using NAT table. So nothing else can use UDP properly. That DNS request to port 53 has a source port of 42981 (for example), and the switch gets the reply.
I'm not sure if most routers prioritize port forwarding rules or NAT tables. That's really up to implementation.
It’s about redirecting. Following these instructions means that the Switch, and only the Switch, can receive UDP on behalf of the other network. Which will break a lot of things. Supposedly this also means that more than one Switch on the same network is a no-go.
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