Electrons spiralling along the geomagnetic field can be trapped by a whistler mode wave travelling in the opposite direction if the electrons are near-resonant and if the wave amplitude is sufficient to overcome the plasma inhomogeneity (e.g., geomagnetic field gradient), if any. An additional wave at a slightly different frequency can detrap these electrons even if the additional wave is 20 dB weaker than the trapping wave if the frequency difference is appropriate. The sensitivity of such electrons to detrapping versus frequency is determined for both homogeneous and inhomogeneous plasmas. Such detrapping may play a part in the control or entrainment of VLF emissions by a transmitter wave and in whistler quenching and echo-induced suppression of entrained emissions.