With a really slow D-loop, or maybe tight obstructions behind preventing a fast and large D-loop, there really is no slide of the line hand towards the rod hand after the haul. Tension is maintained throughout the stroke. The D-loop stroke benefits from the haul, but without bringing the line hand back to the rod prior to the forward stroke, the forward haul is going to be quite short.
Once you can double haul, or at least maintain tension, with the continuous motion casts you can really throw some very long lines indeed. It isn’t easy, it’s another step up the steep ladder of casting prowess, but it is worth it.