An example of heads up is when you yell at a person who isn't paying attention and who is about to get hit with a frisbee.
An example of a heads up is when you let your Internet-surfing co-worker know that your boss is about to enter his office.
Play heads-up basketball.
Gave me a heads-up on the new security measures.
Playing heads-up baseball.
Send everyone a heads-up about the inspection tomorrow.
A heads-up posture meant the pilot couldn't pay attention to his instruments.