It occurs approx 1:46:20 into the film, according to my counter. Batman enters the bell tower, hot on the heels of the Joker. He hears the Joker's voice mocking him and then a flunky, the martial arts, Asian flunky does several flips toward Batman and then launches himself feet first at the caped crusader, knive springing out of his boots. batman calmly stands there and then raises his hand and then the padding on the surface of his palm springs forward on a metal arm below his wrist, adding a foot or two to the length of his arm, hitting the bad guy squarely in the nuts and he drops to the ground.
This device has been puzzling me since 1989 because it does not seem to have any purpose to it unless Battyman actually had the following train of thought:
"You know, if an Asian, martial arts flunky ever leaped at me feet first with knives on his boots, I'd be done for! Wait! I know! I'll put a device on my glove that causes the plam of my glove with the weird pebbly surface to extend out about two feet on a metal rod just under my wrist. then I could just hold up my arm between the fellow's legs and his own momentum will carry him into the device and crush his testical like an elephant on a Timex watch! Gad! How do I do it! Alfred, we shall need some materials from Builder's Square!"
I suppose technically, there is no answer to this question as it was just Burton, who doesn't really care about comic book heroes, added a real IITS moment, but it still bothers me like a cancre sore that just won't heal.