The Chinese tourist next to me was doing things I'd never seen with an iPhone. He was using an on-screen button to pull up a menu and perform a series of rapid commands, and moving that button around the screen, all without pressing the home button.

That floating button is called AssistiveTouch, and it's an alternate input method buried in the accessibility settings. It's designed for people who have trouble pressing hard buttons or swiping in a particular way, but here was a guy with no apparent disability or hardware problems using it like a pro.

I had to ask why: He said something about shortcuts being more convenient.

I asked if he knew other people who used it: He said, "everyone?"

He gave me a quick demo (though on camera he was moving slower than before):

When I looked it up, I found that many people in China use the AssistiveTouch feature of the iPhone, and no one knows why.

"Just about every Chinese person I know does this," writes a Shanghai-based user on MacRumors.

"Probably half of all iPhone users I see [in Guangzhou] have the 'Assistive Touch' option turned on," writes Tencent product manager Dan Grover. "Nobody can give me a straight answer on why they, a person with two functioning hands and a full complement of motor neurons, enabled this obscure accessibility setting. Answers range from protecting their investment on the phone by not wearing out the physical home button, to it just being fun to play with when you’re bored."

Many people claim it is a somewhat irrational attempt to protect the expensive hardware.

As Wang Yijie writes on a Quora thread devoted to this question:

It's because of the fear that the home button may be broken. iPhones are not cheap in China so people take care of them while using. Several years ago people began to complain about their home button being easily broken and it has somehow been a widely recognized truth, so even the home buttons are not that easy to be broken, they tend to use AssistiveTouch instead. When you buy an iPhone in China the salesman would automatically turn on this function while helping you to do the settings. I myself have not experienced a broken home button during 4 years with my iPhone 4; however, I did have a broken sleep button from my 3rd year, which proved that the rumours are, in some way, true. So I turned on AssistiveTouch...

This could be a worrying trend for Apple.

"[F]or a company that is looking to China as its largest market it is worrying that the primary interface feature on their flagship product induces a workaround behaviour for perceived risk of breaking," writes consultant Jan Chipchase on Medium.

Still others, including the guy I talked to on the train, claim it is a more efficient input method.

"iPhone's assistive touch has heavy usage with young Chinese bc of a generation that grew up without hard keys. Remove the buttons," tweets Shanghai-based designer Brandon Berry.

"[M]ost intriguingly it suggests that consumers can do without the button," writes Chipchase. "From that starting point new interfaces are born."

In fact, Business Insider tech editor Dave Smith says his (non-Chinese) dad uses the assistive touch feature and considers it more efficient, though Dave can't say the same.

I tried it for a couple of days (turned on via Settings / General / Accessibility / Interaction) and found it fun but not useful enough. I like being able to call up the surprisingly useful Siri without pressing a hard button and this method of going "home" seems slightly faster than the alternative; but most of the menu options won't be useful for most people, and it's a pain to have that button floating on the screen.

Still, it makes me wonder about the future. That floating button could be a lot more useful if Apple packed it with features for typical users — and taking up screen space wouldn't be a problem if Apple got rid of the hard home button and made the screen bigger.

UPDATE: People around the world are misusing their iPhone because they're afraid of breaking it

Disclosure: I'm an Apple investor.