It's a variant of the belt claw technique of arming crossbows. Possible reasons include extended range due to a greater pull and presenting a smaller target profile for opposing crossbowmen.
Bear in mind that military techniques varied throughout Chinese history, and that much of current theory is based on conjecture. Here's what we do know:
- Chinese armies during the Warring States period (the period depicted
in the film Hero) were swelled by the practice of levee en masse, hence the relatively large army sizes of ancient China in comparison to that of Europe.
- The primary ranged weaponry of these conscript armies was the crossbow, which was easy to use, mass-producible, and customizable to fulfill different roles.
- The Qin army practiced ranked crossbow volleys, where one rank fires while the other reloads, similar to Napoleonic infantry.
That's it. While there are a number of texts depicting military strategy, there are no surviving manuals depicting the nuts and bolts (pun intended) of military technique from the period. Much of what we do know of the armies of the Warring States was gleaned from the accounts of Sima Qian's Shiji, which gave only cursory mention to military technique, and from archeological finds from the Terracotta Army at Xian, which is akin to looking at a cup and guessing that I used it to drink coffee when I was really drinking whiskey.
The only evidence that shows that this version of the belt claw technique was used during the Warring States period is the record of the same technique being used during the Song dynasty, which arose more than one thousand years later, and the remains of similar equipment found with the Terracotta Army.