Motor-Imagery-based Brain-Computer Interfaces (MI-BCIs) show promise but face challenges for a wider adoption due to accuracy and usability issues. It has been proposed that BCIs based on median nerve stimulation (MNS) could significantly improve the efficacy of BCIs. Indeed, when MNS occurs during a MI, post-stimulation neuronal synchronization is significantly reduced, potentially aiding MI-BCIs in distinguishing MI with stimulation from stimulation alone. However, MNS-based BCI only actually works under optimal timing conditions (i.e. when the MNS occurs precisely during MI). Understanding how the ERD/ERS components of mu and beta rhythm interact with each other when MNS is close, but not perfectly aligned, to MI is essential. This study examines 14 participants in two motor conditions: MI with MNS and MNS alone, assessing timing effects on EEG activity. The results show that when stimulation occurs before or at the onset of MI, the ERD of MI tends to abolish the ERS of the MNS.