Bariolage

The effect works because the same pitch sounds different on an open string than under a finger: the open string rings with more overtones and no vibrato, so alternating between the two produces a flicker of timbre. The bow does most of the work, crossing strings quickly while the left hand stays put.

The most famous instance is the Preludio of Bach’s E major Partita, where the open E alternates with fingered notes for bars at a time. Orchestrally it appears wherever a composer wants a string section to glitter, the opening of Tchaikovsky’s Fourth Symphony finale is a well-known example.

It is not a marked technique; there is no bariolage instruction. The player recognises it from the notes themselves, which is why it is worth being able to name.