HR refers to the Speed Force as a "Negative World", to which Cisco is surprised that Earth Nineteen has Super Mario. They are presumably referring to the Minus World, a level in the very first Mario video game that is often regarded as one of the most well known glitches in video game history.
Cisco exclaims "By George, I think he's got it!", closely quoting a line popularized in the 1964 film My Fair Lady.
This episode features 6 speedsters, Barry Allen/The Flash, Wally West/Kid Flash, Jesse Wells/Jesse Quick, Jay Garrick/The Flash, Hunter Zolomon/Black Flash, and Barry Allen/Savitar more then any other episode of the series.
Julian classifies Savitar's blade as an unknown material, akin to a Luttinger liquid. A Luttinger liquid is a material of strongly interacting electrons, confined to one spatial dimension. Such materials have indeed unusual properties and clearly deviate from ordinary metallic behavior.
Jesse suggesting that Savitar's shard would find its way back to him, Cisco likened it to the 1999 film The Iron Giant, which ends with the title character's parts pulling themselves together to rebuild him.
The scene where Barry is entering the S.T.A.R Labs portion of the speed force and Caitlin is singing the lullaby "Hush Little Baby" resembles the beginning of a flashback sequence Batman Beyond: Return of the Joker in which Batman walks through a building as Harley Quinn sings the same lullaby.
The Speed Force comments that Jay was getting slow in his "golden age", referencing the period of comics in which Jay originally appeared.
This is the second time Barry goes into the speed force, the first being in the season 2 episode "The Runaway Dinosaur".