It's an AAD (automatic activation device). It activates your reserve if you get above xxxx mph in your rate of descent when you get to a certain altitude. The metal "sensor" thing is actually the cutter. The cutter cuts away your main.
Yes. It is a Cypres AAD. It releases your reserve parachute by cutting the closing loop that is keeping things closed. It senses altitude and rate of descent and of course, saves lives (hence its "medical" purpose ) It "knows" when you are going too fast to be under a good parachute, if any.