This is long shot, and totally off the top of my head, but I suppose that if he was able to keep the warp engines powered to some degree even before launch then the subspace field thus generated would be able to cancel the mass of the ship to some extent. This is part of the basic premise of warp drive: that a subspace field isolates the vessel's mass from the ordinary effects of Newton's physics to the point where its slippery enough for Einstein's physics to not be able to increase the relative mass due to acceleration towards the speed of light (where it would attain infinite mass and therefore need infinite energy to keep it moving.) I imagine that early application of a weak subspace field could isolate the ship's inertial mass to allow a stubby rocket to push too big a ship up to 11.2 km/s/s.