Bladder effect to explain tubing pressure... what did you understand?

The BP company man makes an attempt to explain the pressure reading of the drill pipe. He makes a drawing on the board, and states there could not be any pressure if there was no flow at surface. He then suggested the pressure sensor reading erratic, and located it somewhere midway down in the riser. First of all, drill pipe pressure is measured at surface, not midway in the riser. And if it was measured in the riser, it would obviously read pressure, since there would be the hydrostatic weight of the mud above the sensor. But this is not the point, I was wondering if the explanation made sense to the general audience. I did hear that the "bladder effect" was described as hydrostatic pressure exerted by the sea on the riser. What did you guys understand?