Turbo routes traffic through Opera's servers in order to compress data - if you have limited bandwidth (dial-up or DSL) or a data cap (as in, 10 GB per month). It is not Mini, it does not pre-render content, but it is like several other "internet accelerators" available. Though as far as "spying", Opera Turbo does not compress secure sites at all.
Mini is actually pre-rendered, and as such would need to route secure data through the proxy as well since the Mini client couldn't render it otherwise.