Does TeamViewer default to LAN over WAN?

[Motaro]

For instance, if I connect to another computer in the regular fashion through partner list, and that pc I connect to happens to be on my own LAN, will TeamViewer sense this and send the data over LAN instead of WAN?  I ask because I've connected like this before and during file transfer the speeds seem to be much higher than what my WAN upload speed is.