Retcon mechanics

Retcon mechanics are the result of a unique ability imparted to John Egbert upon his touching of The Treasure, which enabled him to transport himself throughout the narrative of Homestuck, and selectively re-write the metacanon.

While at times these mechanics have far-reaching retroactive effects on the story, granting the bearer of the retcon ability (thus far restricted to John) powers equal to or even rivaling that of the author, the precise scope of these effects seem to vary. Possibly at the discretion of the user (or perhaps from author intercession to prevent things getting too confusing for archival readers), retcons instigated through this effect vary in scope from complete and seemingly irreversible modification of previously published canonical content in the real world, to simply re-writing the internal history of the narrative, most significantly in the case of the retconned Game Over timeline. Often, these effects intermingle, resulting in a high level of Weird plot shit that may be confusing for even seasoned readers to parse.

It should be noted that in instances when the past is altered via retcon mechanics, even if this only effects the chronological perspective from within the narrative rather than changing the meta-narrative, i.e. the actual structure of the comic, the timeline is altered without resulting in the previous reality becoming an offshoot doomed timeline. In fact, reality is entirely altered, with the post-retcon reality consisting a new alpha timeline, and the previous alpha timeline paradoxically erased from history. However, potentially due to the far reaching power of the Furthest Ring to reclaim souls that are otherwise Void, characters who died in the pre-retcon timeline can still exist within the Furthest Ring afterlife. This is the case with pre-retcon Vriska ((Vriska)).

Origin
The retcon ability imparted to John appears to be the promised power granted by the treasure sought by Vriska throughout her journey through the dream bubbles in Act 6. As the weapon was promised as a means to defeat Lord English, who possess unconditional immortality that can only be overcome through exploiting glitches in Paradox Space, the power of retcon mechanics are revealed; their ability to literally re-write the narrative even from the perspective of the readers likely demonstrates that their use, often with seemingly paradoxical results, constitutes a "glitch" powerful enough to erase or otherwise remove Lord English from history.

The Treasure can purportedly be used only twice HS 7952, once by Caliborn/English, resulting in him such that for the second time it could be used. As such, it is unlikely that any character other and John will be able to exploit retcon mechanics, except perhaps Andrew Hussie, who has has already been killed.

Arm Retcon
Upon first encountering the treasure, John placed his hand into it HS 7994, scattering instances of his arm throughout the Homestuck narrative. This retcon affected the real meta-narrative of Homestuck; that is to say previous pages were amended in the real world to include John's arm, where it had been brought into being by a real "retcon". There are 52 such retroactive appearances of John's arm throughout Homestuck.