Powered by a free public measurement network -- the nearest test server is pre-selected, but you can pick any other.
Powered by Measurement Lab (M-Lab), an open, third-party measurement network. See their data policy
The test runs against Measurement Lab's public servers, not against the VPN servers -- so the figure you get is your own line's capacity, measured the same way whether or not a tunnel is up. Run it once without the VPN and once with it: the difference is what the tunnel actually costs you.