turn on a sixpence

1. To turn quickly or with agile precision, especially with limited room to maneuver. Primarily heard in UK. The steering is so good on this car that you can turn on a sixpence, even at high speeds. The striker turned on a sixpence, leaving the defender flailing past her.

2. By extension, to change focus, activity, opinion, or behavior very suddenly or abruptly. Primarily heard in UK. He's usually a nice guy, but his temper can turn on a sixpence sometimes. Politicians have no loyalty to their causes—they'll turn on a sixpence if it means they'll get more votes.