Verb
(computing) To take something as a controlling input datum.
This logic keys off the user's social security number.
To turn off something (e.g. a car) using a key.
She keyed off the ignition and went into the house.
I once took the key off my girlfriend's key ring so that I could surprise her when she got home. So I did this whole romantic setup in our bedroom with flowers and rose petals. She was so mad when she got home, but then when she walked in, she was so surprised. Trey Songz
Children key off their parents' reaction more than the argument or physical discipline itself. Po Bronson