Well, it isn't just smoke and mirrors - it really does compress things. However, how much the compression helps, if any, is entirely dependent on if the contents of the packets are compressible. It won't help watching movies but may help browsing the web particularly if the sites you are visiting aren't compressing compressible contents.
Having this set to adaptive will cause openvpn to periodically sample the stream to see if it is compressible or not and respond appropriately.
Having this set to adaptive will cause openvpn to periodically sample the stream to see if it is compressible or not and respond appropriately.