Not really any spoilers, and a couple of the reviews also mention the tachyon transporter.
Using an example in the plot that won't really give anything away...what if you could instantly jump from one place to another, particularly when in danger, but couldn't be sure if when you teleported that you would be the version that was safe, or the one that wasn't. Maybe it doesn't matter, since you are in both places...but it sure matters to the one that didn't get away.