A memory flash, also known as memory imprint, was a form of psychological conditioning that affected the mental and emotional status of a clone. The process of creating memory flashes included the scanning of the original subject's brain patterns, which would then be downloaded into the clone's mind. Although this procedure provided the clone with the knowledge and experience of its progenitor, the side effects of inheriting the memories of another person's life were instability and ultimately insanity.
After Galen Marek's death in 2 BBY, the Sith Lord Darth Vader endeavored to recreate his former apprentice by utilizing the Timira City Cloning Facility of the planet Kamino. An altered version of the Kaminoan cloning method resulted in the creation of "accelerated clones," thereby producing fully grown subjects in a matter of weeks rather than years. Utilizing Marek's preserved corpse as a source of genetic material, Vader successfully duplicated the pseudo-Sith-turned-Jedi, only to discover that the procedure was flawed and the subjects unstable.
Aside from the inherent difficulty of cloning a Force-sensitive template, the use of memory flashes undermined the psychological well-being of various clones. Vader wanted the new Starkiller to possess the original apprentice's skills; where Marek learned how to utilize his Force powers after years of training, the same knowledge could simply be downloaded into his clone via memory flashes. But due to the fact that the clone also inherited the memories of a life that it never actually experienced, the subject ultimately succumbed to mental instability. As a result, Vader repeated the procedure numerous times, passing over one failure after another in the hope that a stable clone would finally be created once the cloning process was perfected.
After six months of trial and error, Darth Vader secretly possessed many aberrant clones of Galen Marek, including a more stable version whose progress attracted the Dark Lord's notice. Vader hoped that the project had at last made its first successful attempt to recreate Starkiller, but the latest clone proved unable to overcome certain memory and emotional imprints inherited from his genetic donor, much like other test subjects who failed to pass their trials before him. Realizing that the clone possessed Marek's emotional attachment to Juno Eclipse, Vader deemed him yet another failure in a long line of disappointments. Before the clone could be terminated however, he managed to escape his master in order to find Eclipse.
Around the same time, Darth Vader also oversaw the development of another Starkiller clone, one of who proved even more promising than all of his precursors. While he too was initially confused by the memory imprints, the clone was driven by a desire to succeed where the others had failed. He strove to prove himself in Vader's eyes so that no more versions would come after him and after many trials, the Sith Lord claimed the dark clone as his new secret apprentice. Ultimately, it was the clone's ability to compartmentalize the memory flashes that contributed to his success. By separating Marek's memories from his own experiences, the Dark Apprentice freed himself from his template's emotional attachments, thus preventing him from becoming another clone controlled by the imprinted identity of Galen Marek.
A cloning technician worked with another of Galen Marek's clone, Subject 1157. 1157 received memory imprints as well, and the resulting visions caused 1157's body to tremble and also worsened his temperament. The cloning technician tried sedating 1157 to control the tremblings as the memory flashes were imprinted.
In addition to Marek's clones, Darth Vader also ordered the creation of accelerated clones of Boba Fett, but without the cloned bounty hunter's knowledge. Xasha, a female mercenary and Fett's occasional partner, provided Vader's cloning technicians with Fett's genetic material and the scanned brain patterns that would be used to develop memory flashes. But the Fett clones were unstable, just like the Starkiller batch. The Imperial scientist who oversaw Fett's clones concluded that the subjects would be fit for service as clone stormtroopers for a short while before their minds completely degraded into insanity.
Memory flashes, or memory imprints, were first mentioned in the comic adaptation of 2010's Star Wars: The Force Unleashed II multi-media project. Subject 1157, who received memory flashes from Starkiller, was an ambiguously canon character in The Force Unleashed II Official Website in the lead-in to the release of Star Wars: The Force Unleashed II.