I think (I'm sure someone will correct me if I'm wrong) it measures the temperature of the exhaust gases to make sure the turbo isn't running too hot.
If it is running too hot or the sensor is faulty, then it pulls back timing and puts the engine into a sort of limp mode so the turbo doesn't melt! That's my amateur understanding anyway.
I think I might have to buy new from the dealers but it's so expensive, and just found out there's 2 of them!