Aroduc famously dislikes flashbacks. I've never entirely understood that, because I have a more moderate view on them (as I do on most narrative devices); most of the time I just go with the flow, although I admit that some of the long ones in shonen fighting shows have gotten to me (Naruto's, say, where we might spend an episode or several in one). But recently, writing about Space Battleship Yamato 2199's episode 14 caused a little coherent light to go on in my head about the problem with flashbacks.

Put simply, flashbacks stop the current action in the show to divert us off sideways. We were in the present day with something interesting happening and now all of that gets put on hold while we traipse off to the past for a while, where we have to establish context and build up momentum to do stuff in the past-story. Then afterwards we come back to the present time and have to pick up the momentum of the action all over again (and to remind the viewers just what was going on in the present day before we dropped it for a while). At a minimum the show loses some of the impact from the present day action that was interrupted, because viewers just can't sustain that impact and momentum over the interruption; it falls out of our heads to be replaced by whatever's going on in the flashback. At the worst the show loses a major amount of energy as it thrashes back and forth between the past and the present.

(There are minor spoilers for Yamato 2199 episode 14 here.)

All of this points to why the 'flashbacks' in Yamato 2199's episode 14 work so well: they're actually part of the action in the present. While the flashbacks are of the past, they're being experienced by the characters in the current time. There is no diversion from the action because the characters going through these flashbacks and reacting to them in various ways is the action. The whole thing flows as an unbroken strand from the start of the episode right through to the end, with nothing to yank us out and dissipate the momentum.

I'm sure that there's a bunch of structural techniques that can do similar things, it's just that I haven't had them rubbed in my face so vividly. After all, flashbacks are in theory just one form of having multiple narrative threads running simultaneously as in eg Kyoukai no Kanata episode 10, where in fact one of the story threads was a flashback.

(All of this must be well known to people with actual experience in fiction writing, story structure, and so on. I'm a naif here, though, so I get to work this stuff out for myself when I stumble over it.)