The Private Bench consists of a bench covered with retractable metal spikes and a coin slot. If you want to sit down on the bench without an array of spikes jamming you in the keister, you’ve got to pay about 70 cents. This causes the spikes to retract so you can sit on the bench, and also activates a timer.
Shortly before the timer expires, you’re warned by a buzzer to get up; then, the spikes shoot back up. Just make sure you don’t fall asleep.