Apple already has one, Microsoft and Google say they'll build one, Minnesota will demand it from next year and it could soon be the law in California and maybe nationwide. The smartphone kill switch appears to be on its way to every handset sold in the U.S. so what's all the fuss about? Here's a look at the main points of the technology.

What is it?

For more than a year, law enforcement officials across the U.S. have been pressuring the telecom industry to do more to combat smartphone theft, and the kill switch has been proposed as the answer. It's a piece of software installed in every new phone that can disable a stolen handset.

The laws don't target tablet PCs, basic cellphones or other devices with cellular connectivity.

Why is it needed?

In the last few years, the number of violent thefts of smartphones on the streets of major U.S. cities has been rising. Some estimates say one in three thefts in the U.S. involve a smartphone. Thieves snatch phones from the hands of victims as they walk down the street or sit on public transport and then dart away. A sizable portion of crimes involve people being threatened with knives or guns, or victims are assaulted.

Police believe that if phones can be disabled, they'll become much less valuable on the secondhand market and the incentive for theft will drop considerably.

How will it work?

If your phone is stolen, you or someone you have authorized will be able to call your carrier or use a website to send a "kill" signal to your phone. That signal will lock the device and, if you choose, will also delete personal data. The kill switch will "render the device inoperable on the network of any provider of commercial mobile service or commercial mobile data service globally, even if the device is turned off or has the data storage medium removed," according to the federal proposal.

The only way to revive a locked phone will be with a password supplied by the phone's owner.

When will it begin?

Minnesota's law and the proposed California legislation both mandate a kill-switch for smartphones that are both sold in those states and manufactured after July 1, 2015. Pending federal legislation says Jan. 1, 2015, but that's likely to be changed as it makes it way through committees.

In Minnesota, the software must be installed or available for download, in California it will have to be preinstalled on new devices.

How much will it cost?

The Minnesota law and the proposed legislation in California and at the federal level mandate it must be available at no extra cost to users.

Do I have to have it on my phone?

No. Minnesota's law says it should be installed or available for download. California is mandating the software be on new phones but users will have the ability to disable the feature, but it must be enabled by default. By having it opt-out rather than opt-in, law enforcement believes many more people will leave it switched on, so the chance that any given smartphone will be protected will be much higher.

The deterrent aspect of the kill switch relies on this numbers game: If a phone is likely to have the software, thieves have less incentive to steal it. If it likely doesn't, the chance it will be stolen goes up -- or at least that's the theory.

What about Find My iPhone or Google's Android locator?

Built-in tracking services can help locate a phone and wipe its memory if the phone remains online, but all too often thieves switch off a stolen phone and reinstall the operating system. That wipes all personal information on the phone and your link to it. California's proposed law says the kill-switch software must be resistant to such OS reinstalls.

What's the industry doing?

For a long time, the telecoms industry was against the idea of a kill switch. Speaking through its lobbying organization, the CTIA, the industry said a kill switch would make phones vulnerable to hacking. But earlier this year, as legislation looked more and more likely, that stance changed and the CTIA now supports a kill switch.

But the industry is hoping to avoid legislation and make it a voluntary commitment. Previously, it launched a database of stolen phones that could be used to prevent them from being reused with new accounts. However, the database has limited reach outside of the U.S. and many stolen phones are sent overseas.

Will it work?

It's too early to tell, although some early data from New York, London and San Francisco showed significant drops in thefts of iPhones after Apple launched its kill switch. However, the causes of crime are complex and it's much too early to draw a direct link.

But it's safe to say a kill switch won't do anything to encourage smartphone theft.

So, can the government kill my smartphone?

California's proposed law is the only one that specifically addresses this issue. It allows police access to the tool but under the conditions of the existing section 7908 of the California Public Utilities Code. That gives police the ability to cut off phone service in certain situations.

A court order is typically required, although an exception is made in an emergency that poses "immediate danger of death or great bodily injury."

Martyn Williams covers mobile telecoms, Silicon Valley and general technology breaking news for The IDG News Service. Follow Martyn on Twitter at @martyn_williams. Martyn's e-mail address is martyn_williams@idg.com