That's when the killer kicked him and he burst.
3 seconds sound clip from the Se7en (1995) movie soundboard.
You can hear this line at 00:15:00.668 in the Blu-ray version of the movie.
Quote context
[...]
- Ladies and gentlemen, we have ourselves a homicide.
- Killer put a bucket beneath him, kept on serving. Took his time, too.
- Coroner said this could've gone on more than 12 hours.
- Victim's throat was swollen, probably from the effort.
- And there was definitely a point when he passed out.
- That's when the killer kicked him and he burst.
- Come on. Somebody had a problem with the fat boy and decided to torture him. Simple as that.
- So?
[...]