According to the write up in 'Aeroplane' (Feb 2002, page 24) - Jan failed to 'cartwheel' the Hornet in the horizontal plane, even with one engine at full power, and the other throttle closed. However, when he tried the same thing in the vertical plane, he could get a full rotation of the aircraft. Thus the 'Zurabatic Cartwheel' was born - but not yet perfected.