Question: How did the bolt of lightning in Back to the Future actually work?
Answer: It didn't. It couldn't.
off camera what did happen was a second delorean showed up and used mr. fusion to send power to the delorean stranded in 1955.
In the original failed experiment doc and Marty got together to work on plan B. I will call it "the long way home" they simply waited. Marty grew old starting in 1955 marching toward 1985.
But since Doc had a delorean to copy he was able to recreate it sooner, once the appropriate backup material became available. We see them do this in episode 3.
"all the best stuff is made in Japan"
Since Marty is sent back to 1985 he isnt there to create the 2nd delorean; but since there isnt a 2nd delorean to send power through the line to send Marty back to 1985, he is stuck in 1955. Like Schrodinger's cat Marty is existing in two states at once. Only by observing him do we know if he is in 1955, or 1985.
Because the fate of Marty is always, and constantly in question; that allows the alternate time line in back to the future 2.
Biff steals the Delorean, travels back in time, meets himself, alters the future, but then travels back to the unaltered 2015.
When Doc and Marty use the Delorean they travel back to the altered 1985, and doc explains to us why they can not return to 2015 to stop biff.
Biff, though, has traveled back in time, altered history, and returned to the future that does not include his status as crime lord of hill valley meets detroit.
This is a more subtle reminder that doc doesnt know what he is doing.
remember this thing doesnt work
this thing doesnt work
complete amazement when something did what he wanted to do, remember this is after he remote controls a car directly toward the two of them, with no idea what will happen.
"I finally invented something that works"
Doc and Marty actually died when the dog piloted delorean ran them over.
everything we see after that is Doc's imagined after life.
leading towards its climax, where he accepts his fate and travels into the light at the end of the tunnel.