It detected you bump your phone, sent your GPS location to a server, then looked for someone else who bumped their phone in the same location at the same time.
Both phones send a precise timestamp of the "bump" to the server. The server finds bumps that happen at the same time, and "connects" the phones.
If multiple bumps happen at the same time, the server would probably use approximate geolocation or something like that additionally, but the better the resolution of the time stamp, the lower the probability of collisions between bumps.
"precise timestamps" from two phones is a hard distributed systems problem.
If timestamp and geo were the only things that the app used, I wonder if it worked well only because there weren't too many people 'bumping' at the same time from that geo.
read something about it a long time ago, so fwir : the accelerometer gives you a motion angle of the « bump », and finding the symmetrical motion in a given time window gives you the other phone involved in the bump.
I don't think the "angle" of the bump played into it, since accelerometers only give you relative angles, not absolute values. And the compass isn't precise enough.
