What does "fast-forwarding" mean in the context of CPU simulation?
Fast forwarding is used to warm-up microarchitectural state (caches, branch predictors, etc.) in preparation for more accurate simulation of a particular section of interest within an application. Since microarchitectural state can depend on operation ordering (e.g., cache replacement for data accesses and interactions between threads), speculation (e.g., instruction caching behavior), and even timing (e.g., thermal behavior), this is not a perfectly accurate warm-up, but "all models are wrong".