What kind of delivery is Nathan Lyon's "Jeff"?
In the recent Test series against South Africa, the Australian right arm off-spinner Nathan Lyon occasionally employed a delivery that spun the other way (away from a right hand batsmen instead of towards). I understand he has named this delivery "Jeff" for unknown reasons. There has been some sparse media coverage of this, see for instance here, but I have not managed to find any suitable footage to work out how he is doing it, such as a super slo-mo zoomed in view of the bowling hand or any articles explaining the details.
There are several ways off spinners can get a ball to go the other way. The most famous is the 'doosra' employed most notably by Muttiah Muralitharan, which requires extremely flexible elbows and wrists and long fingers in order to rotate the hand around to impart spin in the desired direction. No non south asian player has ever been able to deliver this ball legally due to the different physiology of Anglo/European players, so I doubt that this is it exactly but a modified version something along these lines might be possible.
The other option is to use the fingers to flick the ball the other way, almost like a regular leg break but using the middle finger to impart spin instead of the whole hand and wrist. Ajantha Mendis uses this and calls it a 'caroom' ball, while Ravichandran Ashwin calls it his 'sudoku' ball. I believe that Jason Krejza has also experimented with this (and probably a number of other spinners as well).
So, does anyone know if "Jeff" falls into either of these categories or is it something else? It has also been described as a 'backspinner', which makes it more likely to be the caroom ball style delivery, or some variant of that.
I'm really looking for a definitive answer here, if one can be obtained, rather than speculation or an explanation of different possibilities, which is why I've outlined at least some of them myself here in the question.