In Sburb, a Scratch is an event that occurs in certain sessions. Essentially, it is a mechanism by which a session, and the universe tied to it, can be reset to an indeterminate point in the past. The process releases a great deal of temporal energy, and the universe will then redevelop differently and eventually produce new Sburb sessions possibly very different from the one in which the Scratch was initiated. It is a last resort for a session doomed to failure, with the intention being to attempt to restart with more favourable conditions.
The Hero of Time in each session is given a device to activate a Scratch, although it is rarely used – Few sessions ever survive long enough to do it. No fan examples have yet emerged, but in the canon kids' session (hereby refered to as B1), the device the Medium has produced for activating the scratch is the Beat Mesa in the Land of Heat and Clockwork, a giant record turntable.
The construct must be scratched (literally) with a pair of very sharp needles (the Quills of Echidna) to activate the process. The involvement of the Quills in the Scratch may explain why in both cases Skaia displayed both the Time and Space symbols spinning as the construct approached. The scratched device rises to Skaia and sends out a request to Skaia to change the conditions of the session, along with the temporal energy required for Skaia to make this change. Skaia's influence however is limited, and it can only do so much. It has full control only over the defense portals, and can reconfigure them to lead to different points in time. As such, it changes the history of the host universe by sending ecto-babies to a different arrival time. This has occurred in both of the known canon sessions. When the original troll session was doomed to failure, its players Scratched it, resulting in a new session in which they became the ancestors of the twelve player trolls who, in the original session, were their ancestors. Similarly, the kids' Scratch has put their ectobiological parents (Nanna, Grandpa, Dave's Bro and Rose's Mom) in their places as the players Jane Crocker, Jake English, Dirk Strider, and Roxy Lalonde. When the kids initiated the Scratch in their session, past Skaia manipulated the defense gates' target location to change the history of the post-Scratch universe. In a way, it is like creating an alternate timeline inside the alpha timeline, meaning that the Scratch was destined to happen. When the original troll players initiated the Scratch, it allowed them to be born in the other session, sort of like a backwards working of the kids' Scratch. How this is possible, we don't know, but it is suspected to be related to the Session Glitch.
The phrase "to start from scratch" refers to the act of starting something over, from the beginning, where an initial attempt has failed and the following attempt is made with nothing carried forward from the first. It originated in the late 1800s in reference to the boundary or starting line of a sports game, which was often physically scratched into the ground. The Scratch may also be thought of as analogous to a record scratch, a destructive sound produced when a record is forced to run backwards and then continue forwards from an earlier point. The record scratch is portrayed quite literally with the Beat Mesa, and symbolically with Disc 2 of Homestuck. Scratch may also mean to destroy or undo (as in the phrase "scratch that"). This can reflect how the destruction of the universes created and gave power to someone who would ravage everything in sight, including the universes themselves. In billiards, a "scratch" is a term for a foul, most often for the act of pocketing the cue ball. After a scratch, the cue ball has to be "reset" to its starting position. While scratching is to be avoided most of the time, there are times for which it might be prudent to do so. However, a player that scratches while pocketing the 8-ball immediately loses.