He says something in regards to "I never thought of space as the thing that's moving." Implying the theory that the ship isn't actually traveling fast than light, but that space warps around the ship to allow for travel great distances very quickly. The fact that one of Starfleet's top engineers didn't realize this is surprising, but we'll ignore that!
I took it more to mean that the transwarp beaming problem was soluble in a different mathematical framework than the one he'd been using. Things like motion are defined differently relative to different coordinate systems, and sometimes a physics problem is more mathematically tractable in one coordinate framework than another.