Brakes more brakes and PCS

Pre-Collision System on the package V. Yea, I've experienced the momentary loss of deceleration over bumpy surfaces. At least in my case this is not really that big a deal. The other day I am pretty sure the PCS was activated on my car. This was a big deal and could have caused a bad rear end collision. Going about 50 mph I hit a steel plate put down to cover road construction. I knew the plate was coming and took my foot of the gas. When the front tires hit the plate I remember thinking I wish I hadn't hit it that hard. What happened next was perhaps the most disoriented I've been in a car that did not involve bending sheet metal. The car decelerated so hard I involuntarily closed my eyes. I mean decelerated hard. Somewhere from the dash there was an alarm beeping and I realized the seatbelt retractors were releasing. Never felt them tighten but they obviously had. During the event my first thought was the engine had seized. The deceleration seemed to come from the center of the car, just from the front and not the brakes. The whole thing could not have lasted for more than a couple of seconds and the car did not come to a full stop before the computers gave me back control. After it was over my first words were "Did I do that?" I have thought about that long and hard. The answer is no way. Does anyone know enough about the functionality of the PCS to say that it was not responsible? I did discuss this with a service writer at the dealer and they claimed no knowledge of this or any other Prius “braking†problem. Ha. Overall I love the car and understand that it is my choice to be driving a car with cutting edge technology.