The juxtaposition of initials for two phrases that allow for an addition to what has been said before, playing on the difference between the formality of "Post-Script", typically seen in print, and the informal use of the initials for the more conversational phrase "By The Way." This hybrid abbreviation is contextualized as modern text-speak, rendering both formal and informal as neo-informal. The phrase is also an example of intertextual irony. Spoken aloud, it should be said "P.S., by the way." Psbtw, this can be annoying and should be used sparingly.
Psbtw, I think I know what you mean about that thing.