This paper presents a theoretical comparison of power swing characteristics between conventional synchronous generator (SG)-based systems and grid-following-voltage source converter (GFL-VSC) systems. Unlike conventional SG-based systems where the swing dynamics are characterized by the change of physical angle between different power sources, the swing characteristics in GFL-VSC systems are influenced by the control dynamic of the VSC, and the loss of synchronization is characterized by the divergence of the output angle of the phase-locked loop, which is control-dependent. Building on these insights, this paper further explores the performance and efficacy of impedance-based swing-detecting schemes at GFL-VSC systems. To validate the correctness of the comprehensive analysis, different simulations are carried out using the PSCAD/EMTDC platform, both in single-machine and multi-machine systems.