demagnetize

When you rent or buy a DVD, the checkout clerk rubs it across a pad to demagnetize the metallic glass so it won't trigger an alarm when you leave.

当你租或者买DVD光盘时，收银员会把它滑过一个垫子使其中的金属玻璃退磁，所以在你离开的时候不会引起警报。