Refer to this "SoftEther VPN has the UDP Acceleration Function. If a VPN consists of two
sites detects that UDP channel can be established, UDP will be automatically
used. By this function, throughput of UDP increases. If direct UDP channel can
be established, direct UDP packets will be used. However, if there is
something obstacles such as firewalls or NATs, the "UDP Hole Punching"
technology will be used, instead. The "UDP Hole Punching" uses the cloud
servers which SoftEther Corporation operates on Internet. UDP Acceleration can
be disabled anytime by setting up so on the VPN-client side."
source: github