on a sixpence
1. With agile precision, especially with limited room to maneuver. Primarily heard in UK. The striker stopped on a sixpence, leaving the defender flailing past her. The steering is so good on this car that you can turn on a sixpence, even at high speeds.
2. In an instant; very quickly, rapidly, or abruptly. He's usually a nice guy, but his temper can turn on a sixpence sometimes.