What's causing the loop? It turns out that, besides getting in a workout every day, Renton is the inventor of the ARQ of the title, which Hannah helpfully describes as "an unlimited energy machine that also produces unlimited time." The machine looks like the world's most expensive metal rolling pin. It seems that, according to Renton, the bad guys "want the ARQ for their war machine," but every time he re-spawns like a video-game character at the same time and in the same location, he learns that people around him are not who they appear to be.
If you like your sci-fi movies packed with jargon (e.g., ARQ stands for "arcing recursive quine"), frantic running, and a solid twist every 15 minutes, ARQ will be comfort food. It's perfect for your friend who owns the complete Sarah Connor Chronicles on Blu-ray. Like the big machine the movie gets its title from, the plot keeps spinning at maximum efficiency. Live. Die. Repeat.