The number of iPhone thefts is down significantly in three major cities since the introduction of Apple's Activation Lock, aka the "kill switch."

The iPhone has always been a target for robbery, and thieves were just as excited as buyers when the iPhone 6 came out last year. But they've learned: Stealing an iPhone these days is a waste of time.

Apple put Activation Lock on phones, iPads and iPod touches running iOS 7 or later. It allows you to remotely lock your phone if it's lost or stolen - and it can't be unlocked without your Apple ID and password.

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"Whoever steals it can't do anything with it without this special unlock code," explained CNET senior editor Jeff Bakalar. "It's different than that lock screen code, this is something that completely renders the phone useless and it's of no value to someone who steals it."

"If [a] phone's stolen, it's basically a paperweight," he said. And who wants to steal a paper weight?

In the 12 months after Activation Lock was implemented in September 2013, the number of stolen iPhones dropped by 40 percent in San Francisco, 25 percent in New York and 50 percent in London, according to city reports.

San Francisco District Attorney George Gascon says the kill switch is curing the epidemic of iPhone thefts that his plagued his city.

"We immediately saw a modification of behavior on the street - as thieves were taking phones, they realized they could not put the phone to use," he told CBS San Francisco.

California passed a law that goes into effect July 1 requiring that all smartphones sold in the state be enabled with kill switch technology, and Gascon hopes the problem shrinks even further once all phones are covered.

"This device is now full of so much personal data - medical data, banking data - that it would have a bad impact if somebody else gets a hold of it," he said.

Activation Lock also gives users the ability to remotely wipe their phones of all personal information.