How to do a Flying Knee
A flying knee (known as hanuman thayarn in Muay Thai, and sometimes called a jumping knee) is a knee strike very similar to a front knee, except that it is performed in stand-up fighting by jumping, and often by rushing towards the opponent. A more reckless application of the flying knee strike can be applied by rotating the body so that the side of the knee strikes the opponent, used more as an offensive pushing attack rather than a concussive KO attack. Generally, flying knee strikes can be effectively applied when the opponent is off-balanced, recovering from previous strikes, or as a counter to a strike by the opponent. It can also be used as a follow-up maneuver after delivering a particularly incapacitating strike.
The flying knee can be done by first starting the running motion and then pressing the RT + A or B button (R2 + X or O or PS4).
If you want to swap those out for running punches you can do so by using the X or Y button instead of A and B buttons. (This would be RT + X or Y on Xbox) R2 + Square or Triangle on PS4.