When you wash dishes at a restaurant, on a night that has more customers you will wash more dishes than usual, but get paid the same amount of money.

Isn't this the basic idea, since the time is already bought and paid for, its in the interest of a capitalist to get as much as they can from the worker in that hour of work. It's been happening in America a lot recently, cuz workers are quitting jobs and the ones left behind are worked harder.