Learning to Beatbox? Here is the Human Beatbox Sound Archive entry for the K Rimshot.
Name: K Rimshot
Family: K Based Snares (K)
Description: This sound is made without bringing air in or out of the lungs. Instead, it is made by stopping airflow completely by putting the tongue against the soft palate, then pulling creating suction by bringing the glottis down. Then, a quick burst of air is brought past the tongue, but not into the lungs. Air can be pulled in down past the center of of the tongue, or to one or both sides of the tongue. This sound is an implosive.
Other Links: About the Human Beatbox Archives and Notation System