It will unlock the halter around your son's neck and will free the trolley.
11 seconds sound clip from the The Abominable Dr. Phibes movie soundboard.
You can hear this line at 01:21:54 in the Blu-ray version of the movie.
Quote context
[...]
- Are you receiving me, Vesalius?
- That is an x-ray of your son's rib cage.
- You will see from it that a tiny key has been lodged close to his heart.
- It will unlock the halter around your son's neck and will free the trolley.
- In a few moments, acid will be released into that tube. It will creep down slowly, inexorably.
- It will take six minutes to reach the outlet over his face.
- Exactly six minutes, Doctor.
[...]