When somebody says or does something stupid, "hands" is called on them. The person that called it gets to slap the other person's hand. If somebody calls "brakes" then it cancels out "hands", therefore one would call "hands, no brakes" to prevent that from happening.
Jonathon: Shit, I don't know what to write in this example box!
Bumm: Hands, no brakes! *slap*