The operative word is “low-fat.”
An operative motive.
An operative dose, rule, or penalty.
When a new solar-powered car is now functioning and runs successfully, this is an example of a situation where the car is operative.
When a law is in place banning watering your lawn, this is an example of a time when the water ban is operative.
When you want to emphasize that someone is really very wealthy, this is an example of a situation where you might say "the operative word is rich."