Arccos sensors are shot impact detectors. Once installed on your clubs, they detect impacts when you hit a golf ball.
Each Arccos sensor also has a Bluetooth transmitter inside of it. These Bluetooth transmitters pair to your iPhone (much like how a mobile phone pairs to a car and allows hands-free calling through a car stereo). Each club is paired to your iPhone as a separate and unique device. This allows the Arccos app to know which club you used when you hit a shot.
When a sensor detects an impact, it sends a signal to the iPhone. That signal says something like, "I'm a 7-iron and I just hit a shot." When the iPhone receives that signal, it uses its GPS capability to make a note of exactly where that shot occurred. The Arccos app then overlays that data onto a map of the hole you're playing. Arccos knows where the fairway and green and bunkers are located, so it knows how to categorize your shot as a drive, approach, chip, putt, etc.
Because Arccos sensors detect impact, they can register shots from unintentional impacts. For example, dropping a club or carrying two clubs in your hand and having the heads clank together. Minimizing these unintentional impacts will make editing your rounds easier.
Because Arccos utilizes the GPS functionality of the iPhone to know where you hit your shots from, it is imperative that you keep the iPhone on your person when you play. If you leave the phone in your cart, when you hit a shot the iPhone will take a GPS reading from wherever the phone is instead of where you actually are. For example, if you leave your phone in your golf cart and go toward the putting green and take a chip and a putt, Arccos will think you hit that chip and putt from wherever your cart was.