"Prochnost" means "strength" and is a word Oliver learned from long-time friend Anatoly Knyazev who first appeared in episode 2x4 "Crucible;" both men have been known to use it often as a toast and salute.
Mia tells Oliver that she doesn't cook much, to which Oliver quips that it makes sense; referencing that Mia may have inherited Felicity's bad cooking skills.
Like the previous episodes of the season, this episode has also many references and callbacks to previous seasons, mainly from the flashbacks Season 5. *Oliver and Mia shooting arrows in tennis balls is one to the first season. *The name of the episode is likely a callback to "Crucible", in which one of Anatoly's first words to Oliver was "prochnost". Anatoly and Oliver also talk about Anatoly's first lesson to Oliver, which was "living is not for the weak", that also happened in Crucible. *John meeting Roy is almost the exact same as Thea's meeting with Roy in "This is Your Sword". *When Anatoly and Oliver first meet, he jokes about not punching him, which he did upon first meeting him in "Bratva". *Oliver first fighting a man bigger than his size, ending tied-up in a chair and getting interrogated by Bratva is very similar to what happened in "Legacy". When Oliver and Mia talk about getting out of the chair by dislocating thumbs, Oliver quotes Anatoly from the same episode, saying "it's a useful technique, if getting tied-up in chairs will become habit". Mia having to fight four Bratva thugs and ring the bell is same Oliver had to do in "The Recruits". *Kapiushion is the nickname Bratva gave Oliver, originally in it was Kapot, but was changed, due to latter meaning "car hood". *Anatoly says that both him and Oliver have matching tattoos, with Oliver replying not anymore, referencing the events of both flashbacks and present day of "Kapiushon". *William says he met Mia in a fight club, referencing the events of flash-forwards in "Unmasked".
Oliver mentions Anatoly having gone to the Maldives, but Anatoly mentions that with Diaz dead, he had no real need to stay in hiding. This is a reference to the season 7 episode Due Process.
When Diggle goes looking for Roy, he finds him working as a car mechanic under the name Jason. This is both a reference to when Roy worked as a car mechanic under a new identity after faking his death at the end of Season 3, and to Jason Todd, the 2nd Robin, who in the comics is first introduced trying to remove a wheel from the Batmobile and who became close friends with Roy Harper.