Even the best multi-way earpieces do not provide a great deal of bass extension. Some drummers find it difficult to adjust to in-ear monitoring because they are used to physically feeling bass frequencies, both from large bass guitar rigs and to some extent the sound of their own bass drums in wedge monitors.
A bass shaker is essentially a type of subwoofer which transmits low frequencies by direct contact with the drummers stool.
These low frequencies (5-200Hz) are transmitted through the stool to the drummers body. This can dramatically help the interaction of bass and drums, as the drummer feels every note the bass player plays without the necessity for high volumes on stage. Depending on the model, the shaker can either be fed a full range signal or a low-pass filtered sub out signal.