A Bartok
pizzicato is a special pizzicato technique that was brought into
classical music by the
Hungarian composer Bela Bartok, although it's not believed that he invented it himself.
A Bartok pizzicato is used on bowed string instruments (that is violin, viola, violoncello and double bass) in classical music.
A Bartok pizzicato is a very strong pizzicato that results in the string striking the fingerboard, played by drawing the string directly upward from the fingerboard with a lot more force than is used for a normal pizzicato. The resulting sound is hard, metallic and resembles a whip or a slapstick. The sound has some pitch, although little. A Bartok pizzicato is therefore usually notated with a determined pitch, although it can be hard to determine.
The notation of the Bartok pizzicato is a small ring with a vertical line through the top.
As a rule, the volume of a Bartok pizzicato increases with the length of the string being drawn. This means that a note played on an open string (not pressed down by the left hand) generally gives the most volume, and as you move higher up the string (toward the bridge of the instrument, higher pitch) the volume diminishes. At the highest pitches, a Bartok pizzicato won't sound at all, resulting in a normal pizzicato instead.
Bela Bartok lived from 1881-1945. He didn't use the Bartok pizzicato all that much, and not in every work. After his works, other composers adopted several of his techniques, among them the Bartok pizzicato. In the following decades, it was used extensively and is now considered a normal technique in contemporary music, but is (barring Bartok) almost exclusively used in music written after 1945.