Hysteresis.
The Casimir effect can be attractive, repulsive or, in some situations, neutral. The paper I just looked at exploited that. It allowed a chamber to expand via the Casimir effect until the effect was neutral and then shrunk it entirely in the neutral phase.
This may be strange, but this is a quantum system and it doesn't have to conform to the expectations of a macroscopic being who is used to the classical world.
That said, the paper's own author was highly sceptical of the maths. And it ignored real world things like friction. But it's possible, even with friction, you might be able to get out more than you put in. We regularly steal the earth's rotational energy; it's possible we could steal energy from the vacuum.