Abstract The interplay between externally induced locked modes and the plasma toroidal rotation is studied in JET L-mode plasmas, through the analysis of shots in which the non-disruptive compass scan technique [Piron L. et al. Nuclear Fusion 64 (2024) 066029] was performed. In these experiments, an external magnetic field perturbation was applied up to the onset of a locked mode, with toroidal mode number n = 1, at constant plasma density. In Ohmic plasmas, during the execution of the compass scan, it is observed that the toroidal rotation brakes in a wide range of the core. In the case of Neutral Beam Injection (NBI)-heated shots, the rotation braking is significantly reduced. The experimental analyses have been interpreted by the RFXlocking code adapted to the JET tokamak [Zanca P. et al. Nuclear Fusion 55 (2015) 043020], which also allowed to investigate the roles of the electromagnetic force and of the neoclassical toroidal viscosity, in the rotation braking phenomenon. Plasma rotation has been analyzed also during a density variation in the absence of external magnetic field perturbations, highlighting that a profile transition occurs in the core.