In revolution, perhaps, have they a braking effect? (5)
I believe the answer is:
discs
If this is the answer, I'm afraid I cannot explain it.
Can you help me to learn more?
(Other definitions for discs that I've seen before include "Type of recordings" , "Flat, thin circular objects" , "Layers of cartilage between vertebrae - they may slip" , "Records" , "They're round and flat" .)