Our previous transcranial magnetic stimulation (TMS) studies showed excitability changes in the corticospinal system (CSS) of the relaxed upper-limb when visual stimuli are targeted by eyes alone. CSS changes are compatible with an implicit program for an aimed movement of the hand to the same target of gaze. Thus, eye and hand motor systems are coupled even when only one effector is engaged. We investigated the nature of this coupling by conducting two experiments to assess whether CSS changes are induced whenever a shift of spatial attention is occurring, or are tied to the actual execution of an eye movement. CSS excitability was studied by applying a single-pulse TMS on the left motor cortex and motor evoked potentials were recorded in three muscles of the contralateral relaxed upper-limb. Covert shifts of spatial attention were ineffective to trigger excitability changes of the arm motor control system which, conversely, occurred when eyes overtly moved to an attentive stimulus (prosaccade). Notably, no modulations in CSS excitability followed eye movements driven by cognitive factors (antisaccade). These findings unveil the motor nature of the signal coupling the hand motor system to saccadic eye movements, although not in a slavish manner. On the contrary, behavioural contest, task familiarity and stimulus-response compatibility determine whether an implicit motor coupling of eyes and hand has to be switched on.