Actually, it would seem he is not using the turbine engine to drive the prop. It is actually jet propelled. Crazy Missourian, this guy.
http://www.franklinairshow.com/
I initially thought the same thing, Bruce, due to the downward angle of the jet exhaust (obviously necessary to divert hot gases away from the Waco's fabric covering). I figured that would be problematic in terms of thrust vectoring, but the web page specs on the Waco seem to indicate that the jet is in addition to the Waco's normal Pratt and Whitney radial.