Although they called it "neurons and mesons," which made me wince every time, since mesons are subatomic particles, not components of human biology (except insofar as they're found inside all of our atomic nuclei as exchange particles for the strong force). It's like talking about the blood flowing through a person's arteries and batteries.
No, that's not how it was presented. The show's inane idea of "string theory" was that a person's life was a "string" from beginning to end and you could bunch it up so the parts of it connected. The intent was that the method of time travel was fundamentally dependent upon the traveler's own lifeline, that it was like a cable car in that you could go anywhere up or down the length of the cable but not beyond its terminal points.
Granted, though, in "The Leap Between the States," it was stated that Sam leaping back into his Civil War-era great-grandfather was partly due to a genetic link and partly due to an "error" in Ziggy's programming. So by that point the show was playing fast and loose with the original theory in order to justify the story. Though I often suspected Ziggy of having more control over the leaps than she admitted.
