From our pals at the ever-great site, Cinephilia and Beyond, comes Michael Chapman talking about the use of slow motion in Raging Bull:

We were pretty precise about what we wanted and we had all sorts of rules, you know, the actual boxing would all be at 24 frames, but other times it could be other… when it wasn’t just the boxing, or there’s some famous shots where it’s in 24 frames, and then you go to 48 frames while Jake walks away in the neutral corner and he’s breathing, and he comes back to 24 frames when he’s going to fight again, but it’s all in one shot, and we did… that was okay, because he wasn’t actually boxing when he went to 48 frames, and we did it with a… really just by hand, and now you can coordinate that and punch it in, but in those days you did it by listening to the sound of the camera changes — speed changing and then opening and closing the diaphragm in… in relation to the change of the speed. But guys just did it by hand; we did it two or three times and it worked out. If you don’t do it right, of course, it… you know, it gets all buggered up. We did it I think every time and it worked out all right, and then occasionally when he’s in the corner, and they’re pouring water over him, we would go to a really 96 or 120 frames and… and really be outrageous, but when they were boxing we made sure they were always 24, except I think like all rules we broke them a little bit in the end, but, anyway, we had very elaborate rules and very elaborate methodology that we worked on all the way through the movie.