The impeller drive used a series of nodal generators to create a pair of stressed bands in normal-space, one "above" and one "below" the mounting ship. Inclined towards one another, these produced a sort of wedge-shaped quasi-hyper-space in those regions, having no direct effect upon the generating vessel but creating what might be called a "tame grav wave" which was capable of attaining near-light speeds very quickly. Because of the angle at which the bands were generated relative to one another, the vessel rode a small pocket of normal space (open ahead of the vessel and closing in astern) trapped between the grav waves, much as a surfboard rides the crest or curl of a wave, which was driven along between the stress bands.
That is probably what gave you your mental image. But note the sentences immediately following:
Since the stress bands were waves and not particles, the "impeller wedge" was able, theoretically, at least, to attain an instantaneous light-speed velocity. Unfortunately, the normal-space "pocket" had to deal with the conservation of inertia, which meant that the effective acceleration of a manned ship was limited to that which produced a g force the crew could survive.
The impeller wedge refers to the impeller bands themselves. The normal space pocket is the volume inside the wedge, which includes the entire ship.
It seems to be saying that the pocket of normal space, including the ship, gets driven forward by the accelerating grav wave around it (the wedge), but the pocket is still subject to inertial forces.