How does netcat-openbsd work with UDP?

On a Ubuntu system I have the netcat(-openbsd) package installed, the one that, when run with no parameters, prints:

This is nc from the netcat-openbsd package. An alternative nc is available
in the netcat-traditional package.
usage: nc [-46bCDdhjklnrStUuvZz] [-I length] [-i interval] [-O length]
          [-P proxy_username] [-p source_port] [-q seconds] [-s source]
          [-T toskeyword] [-V rtable] [-w timeout] [-X proxy_protocol]
          [-x proxy_address[:port]] [destination] [port]

I run this command, which I think listens on a UDP port:

nc -v -u -l -k 1234

Then in another terminal I run this to talk to the listener:

nc -v -u 127.0.0.1 1234

But this behaves very different than I expect: * When I run the second command (sender), the first one (listener) starts printing "X" characters, one per seconds, 5 times. Only then the sender reports a successful connection. (How does it even know with UDP?) * If I omit the -v parameter on the sender side, which I thought only sets the logging verbosity, then the "X" characters are not printed on the listener side and instead the connection is established immediately. * Despite giving the -k parameter on the listener side, once I make a connection to the listener, the listener seems to be "used up". If I kill (Ctrl+C) the sender and start it again, any data sent is lost (does not appear on the listener side). What am I missing? How do I *correctly* listen on a UDP port? **Edit:** I found a way that works well (socat -u UDP-RECV:1234 STDOUT) but I'm keeping this question open because I'm still interested in an explanation.