FAQ for
Irréversible (2002) More at IMDbPro »

During the movie one of the leads searches for the suspected rapist in an underground sex club. When he locates the man a fight between the two ends with a brutal repeated head bashing of the suspect with a fire extinguisher. The scene is hyper-realistic and hard to watch.

The flashing lights are to create a sense of uneasiness again. Gaspar Noe uses low frequency sound to create nausea, headaches, and disorientation in the beginning and uses it again here to make the viewer feel sick after the mood changes dramatically.