We study the motion of electrons trapped in the potential troughs of a monochromatic whistler wave propagating along geomagnetic field lines. Because of the coupling between inhomogeneity and trapping effects the distribution function of electrons at the termination of this triggering pulse can be very different from the distribution at its front provided that the triggering wave amplitude and duration are large enough to trap electrons over more than one trapping length. Particles which are detrapped at the triggering wave termination have been phase organized and may then act coherently for a while, giving rise to an emission with either rising or falling tones, depending on the sign of the inhomogeneity variation. The spatially averaged part of the distribution function of these suddenly detrapped electrons is shown to generate an instability which amplifies previously emitted waves. These results are compared with existing data. Finally, we show that emitted waves can easily trap electrons, thus explaining how the process of emission can be self-sustained.