In "From wormhole to time machine: Comments on Hawking’s Chronology Protection Conjecture" by Matt Visser (http://arxiv.org/abs/hep-th/9202090), he summarizes how "time machines" may be created from traversable wormholes (Morris and Thorne):
"Having constructed a traversable wormhole with time–shift the final stage of time machine construction is deceptively simple: merely push the two wormhole mouths towards one another (this may be done as slowly as is desired). A time machine forms once the physical distance between the wormhole mouths ℓ is less than the time–shift T.
"Once this occurs, it is clear that closed timelike geodesics have formed — merely consider the closed geodesic connecting the two wormhole mouths and threading the wormhole throat..."
It's not clear to me though. Could someone explain why moving the mouths together creates "closed timelike geodesics (curves)"? Would pushing just one toward the other accomplish the same effect (which is what I understood from my question here Wormholes, time travel, and time dilation. Would moving them apart afterward stop the "time machine"? Perhaps the answer is in the math, which is beyond me.