We considered a button but ended up going against it. It doesn't scale - the button doesn't give us enough data. People don't time their presses well enough and if the app becomes successful we'll have a hard time telling apart presses that occur close to each other (both in time and space).

We've also considered a 4 digit pin but that confused users. It turns out that users intuitively understand the shake UX to transfer the contents from one phone to another.

Not to be argumentative but doesn't shaking present the same scaling problems? You can only stack so many iPhones before it becomes dangerous to shake them (for me shaking one is enough!). Why not put a button with a certain timeout, surely users can hit the button within 5 seconds of each other.

We're not talking about scaling by syncing more than 2 devices simultaneously. We're talking about scaling our user base.

The problem is that we can only expect GPS to be accurate to 1-2km and the phone clocks are often off by 1+ seconds. This makes it difficult in metropolitan areas when multiple transfers can occur simultaneously within a 1-2km radius.

Ah, I misunderstood the mechanism, I thought it transferred over wifi, sorry.

why would it be dangerous to shake a phone? it's all solid state materials, it shouldn't cause damage from shaking your phone unless you throw it.

I was joking about shaking one, but stacking two and shaking makes me uneasy because I am paranoid about scratches. And yea, throwing or more simply dropping is a slight concern.

Bumping the phones against each other creates a certain motion signature you can use to identify the pair of phones in the transaction.

I think the accelerometer data is very useful to detect devices.

Isn't this a way to draw attention to the device/app -- a form of marketing in public?

It does look cool, IMHO. I think it would especially appeal (at least in terms of novelty) to non-techies who aren't familiar with the concept of bumping.

just shake it!