This is the udp flavor of the sendUpdate API. Its mostly used by developers who are making action based realtime games where latency is to be minimized. Otherwise sendUpdate (default tcp) is fine. In the case of udp the server doesn't send a response onSendUpdateDone as it does for the default tcp update.
You can read more in the following blog post
To use this, you must also complete a udp handshake with the server. This is initiated through initUDP().