The scenes involving Jet Li fighting himself were shot using Li and a stuntman with a build similar to his. The stuntman's face was painted green so that it could be digitally replaced with Li's face later.
Jet Li uses two distinct kung fu styles when portraying Gabe and Yulaw. The evil Yulaw uses Xingyi Quan, or "Mind-Form Fist" (loosely translated). As can be seen when he is training, it is a very linear, offensive style. Gabe on the other hand practices Bagua Zhang, or the "Eight Trigrams Palm". It is characterized by open palm fighting, with circular footwork. Both styles are two of the three main "internal" styles of Chinese kung fu - the third being Taiji Quan (Tai Chi).
The 'University Medical Center' where Gabe goes for the MRI is the North Hollywood Medical Center where the popular sitcom Scrubs (2001) is filmed.
The scene in which Yulaw is practicing his Xingyi while shackled and jailed is a reference to the famous Xingyi master Kuo Yun Shen who did the same thing when he was jailed.
When Gabe's wife is introduced when she's getting ready to see if Gabe is alright after being attacked, the background sound of the TV is heard playing Destination ultime (2000), one of James Wong's and Glen Morgan's earlier films.