In high school I actually had a friend teach me this in a humorous way. In football we did sprints (the kind where you physically run) at the end. He said -- half ass the sprints. Then on the last sprint, run as fast as you can, you'll have more energy left, and when they see you running fastest you'll get all the credit. He was doing it as a joke, but lo and behold he out runs everyone on the last sprint by a lot, and the coach specifically pulls him aside as an example of how everyone else should carry themselves.
He's one of my favorite pranksters. But needless to say, the football team was not very good.
In the case of football that might actually be a good lesson: save enough energy for the end of the game so that you can play best when the other team is tired. Of course you need to play the whole game, but if you play all but the last 5 minutes working just hard enough to keep a tie game, then you can pull ahead in the end.
This only works in time limited sports though. Most real world situations are not time limited like that and so the advice doesn't apply.
On sprint training. Me and a friend figured out that if you moved your arms fast whilst not running a full sprint it gave the appearance that you were trying very hard and coach didn't give you a hard time. Like most slackers instead of using the trick once in a while we over did it and the coach soon figured it out. The result was even more sprints. Needless to say our team too wasn't too good.
That really only works if the other team is terrible relative to your own. ‘Playing to tie’ a team that is an even match (or close to an even match) results in a competitive advantage for their side.
This does work ok for long distance competitive running, since the factors at play are time and metabolic rate and is missing the strategy present in team based ball sports like football, soccer, basketball, etc.
He's one of my favorite pranksters. But needless to say, the football team was not very good.