This review WILL CONTAIN SPOILERS for the episode to be discussed, if you do not want certain plot points or story spoilt, please do not read further.
The Flash has had some little extra gems in its storytelling arsenal that other comic book shows don't, time travel and parallel dimensions. Both of these adds a whole slew of possibilities for both story telling and character development, and whilst most of season two has thus far been focused on parallel worlds 'Flash Back' uses time travel to great effect in one of the more interesting episodes this year.
With Barry finding himself unable to reach greater speeds in his training to take down Zoom Caitlin points out that Zoom and the Reverse Flash both have an advantage over Barry, their feet are on the ground a lot less than his, Realising that both villains have somehow managed to unlock the secrets to moving at greater speeds Barry hatches a daring plan, travel back in time to the previous year and get Eobard Thawne (posing as Harrison Wells) to help him.
Despite having travelled in time more than once already this is one of the first times that Barry has done so willingly and with purpose. That made this time travel story feel stand out and different enough not to get lost in the other time travel stories, that and the introduction of the Time Wraiths to the mythology.
The Time Wraiths are new, not just to the CW universe but to the overall mythology it shares with the comics. They've never been in the books, so the introduction here was a complete surprise to me. The episode describes them as living within the Speed Force, and that they will stop speedsters who try and travel back in time.
Okay, so that explains why they haven't appeared in Legends of Tomorrow, they only affect speedsters, but why haven't we seen them before? Well, it could be because this is the first time that Barry has travelled back in time without it being an accident. Also, all of the other times he's done so it's only been by a few hours/days, this is travelling back almost a year. Perhaps the further back you travel and doing it on purpose brings about the Time Wraiths.
Whatever the reason we'll probably learn more about them in the future as Thawne appeared to have past knowledge of the creatures and we haven't seen the last of him in the show. Maybe the younger Thawne will encounter Time Wraiths when he travels back to fight Barry in events we've not seen yet. I know, time travel makes my brain hurt sometimes too.
When Barry arrives back in the events of season one he immediately gets into a fight with his younger self that, apart from some dodgy moments of CGI, is an entertaining moment as we watch the Flash fight himself.
The rest of the episode focuses on Barry trying to interact with previous events without altering them too greatly, something that he comes close to failing to do more than once, yet somehow works out well for everyone in the end. So far the only change we've seen Barry's trip cause is that Pied Piper is now a good guy instead of a villain. Being a fan of the comics this chance in Piper was fun to see as there is a run in the comics where he reforms and becomes an ally of the Flash.
Some of the best scenes in the episode have to be Flash coming face to face with Thawne/Wells again. This is different from the version of the Reverse Flash we saw earlier in the season, that man hadn't gone up against Barry before, this version, however, is the one that killed his mother and manipulated Barry's life.
Barry tries to act like his younger self, but even in the scenes when Wells doesn't know who Barry is you can see the looks of disdain and hate on Barry's face when the two of them are together. Once again Grant Gustin does an excellent job at portraying the complex emotions at work in this episode, of a man who wants nothing more than to hurt the monster in front of him but knows he has to hold himself back and pretend to like the man.
Despite Thawne figuring out that Barry is from the future Barry somehow manages to convince Thawne that his plans to return home to his own time succeed and he lets the future Barry live. Why Thawne lets Barry live or simply doesn't use the older, faster Barry to help him time travel there and then isn't exactly clear, other than the fact that doing so would pretty much break the show.
'Flash Back' manages to give us a time travel story that works brilliantly well within its own universe, even when things go wrong our hero manages to nudge events back on course in a way that keeps the time line from completely falling apart, and even gives our heroes a new ally.
As a final quick side note, some people have pointed out how the Time Wraith's face looks like it's wearing a similar mask to Flash and the Reverse Flash. Yes, it does look very similar, and this thing does live inside the Speed Force, so yes I agree with what people are saying, this thing is probably connected in some way to the Black Flash.