Slipforming, I believe, involves the concrete placed in bulk in front of a moving vibrator/screed that maneouvers the concrete into place. In this method, the hollow cores are simply formed in place prior to depositing the concrete. The concrete stays in place and the form moves over it.
Extruded - the concrete is pressure injected through a non-moving form pattern that replicates the cross section of the hollow core plank. The concrete is mixed to a low enough slump to maintain its shape. The form stays in place and the concrete moves along a guide conveyor.