In Wing Chun, if I’m going to kick, I’m going to kick towards an opponent’s centerline—perpendicular towards his right patella. Then I want to immediately punch off. That is the way to break an arm and a leg simultaneously.
If I were to try and kick the opponent’s forward leg, head on, with my right there is not only the possibility of him resisting your attack, you would be putting your balance at risk as well as limiting the potential for success of your attack. If I was to kick with my other leg, the chances of causing serious harm to him would be higher, but you would want to have enough distance between you and him because he will more than likely throw a punch with his other hand (since the other is immobilized).
Remember, You Don’t Have to Break an Arm and a Leg
It’s actually okay if the kick wasn’t successful because you can instantly come off, move in, and break his defense. Remember, you’re attempting to break an arm and a leg simultaneously—it doesn’t mean you have to succeed at both. If I was to repeat the scenario (going for an arm break as well as kicking in for a fractured femur simultaneously), you are likely going to end up completing one of the two. If an attacker was to punch at me, I’m either going to break the arm then kick, or I’m going to pull and kick. You could continue on, breaking the arm with his/her elbow towards the outside, but this would best be accomplished by someone truly adept at Wing Chun. You can, however, add onto it in other ways. After the kick, if you were to pull the attacker to the ground, twisting the opponent’s arm could easily result in it breaking.