footwork is not jumping to the right position after playing a BH form the FH side . most of it is to play with your stronger side ( usually FH ) most of the time and Xu Xin is master of it .
Footwork is not just moving around. Footwork is keeping your body in the right position by moving your feet. If when you are doing forehands from the backhand side and you get moved towards your forehand and you have to cross over, your opponent has broken down your footwork and positioning. If you move to every part of the court and can keep your feet in stance and your torso in position, that is very good footwork. Zhang Jike does not often turn to his forehand from his backhand side. That is because his backhand is soooo good he does not have to. He can pin people to the backhand side with his backhand or make them pay if he is hitting there and they turn to their forehand. But, when he wants to, he can turn to his forehand while on the backhand side as well.
All this is why Zhang Jike was able to beat Xu Xin 4-0 in the WTTC semi-finals. While Xu Xin was running all over the place and getting his feet crossed, Zhang Jike capitalized on when he was out of position. He might be fast enough to get a lot of balls without perfect footwork, but, fundamentally, his footwork breaks down frequently.
If good footwork was just moving to play your stronger side then:
1) Zhang Jike is definitely doing that with his serve return and not getting out of position for the next shot. But Zhang Jike is doing good footwork and not getting out of position, through entire rallies.
2) Ma Long would have really good footwork, because, like Xu Xin, he turns to his forehand a lot also. But, when someone goes down the line on him after he has turned to his forehand, he often crosses over, he often does funny, unconventional things to try and get to the ball, and his footwork becomes sloppy.
Xu Xin moves deep to the backhand side to take his forehand in part because he is not as confident with his backhand. Same with Ma Long. Xu Xin is fast and can get to a lot of balls. But, when he moves, his fundamental footwork often breaks down and he uses last resort foot steps instead of solid fundamental footwork to get balls that are hard to reach. It is very rare to see Zhang Jike have to cross over to get to a ball because he keeps his form in his footwork and gets to so many balls very efficiently.