As global energy systems transition toward high shares of renewable energy, maintaining frequency stability becomes increasingly challenging in the case of the reduced inertia and dispatchability of inverter-based resources. Power generation, including renewable energy technologies as well as thermal power generation, continues to serve a vital role in frequency regulation of power grids but confronts accelerating operational issues, especially when tackling frequent, rapid cycling. Flywheels with their fast response, high power density, long cycle life, and minimal environmental drawbacks, have emerged as promising auxiliary resources for enhancing flexibility in frequency regulation challenges. This paper presents a comprehensive review of flywheel technology development and its limitations, followed by an introduction to the diverse types of grid-scale high-power flywheel energy storage systems. Overviews of the flywheel-assisted power grid paradigm, focusing on advanced flywheel technologies, coordinated control strategies, and economic optimizations in electrical trading markets, are also summarized. The electricity trading market mechanisms, including ancillary service reforms and capacity payments, which reshape power grid balancing by leveraging the role of fast-response storage, are further investigated and discussed. Finally, practical pilot implementations are examined in regions such as Shanxi and Ningxia, China, and Bacon, the United States, demonstrating the efficacy of the independent Flywheel Energy Storage System (FESS) and assisted power generation. This insight expands the research landscape and provides new directions including the interoperability of FESS with low-inertia grids, comprehensive lifecycle assessment, integration within hybrid storage topologies, and the design of investment incentives to promote large-scale adoption.