The roll (a sustained, buzzing rattle) is the castanet's signature effect. Cheap VSTs map a roll to a single sample loop, which sounds robotic. High-end castanet VSTs use granular synthesis or physical modeling to create genuine, tempo-syncable rolls that you can start and stop naturally.
Whether you're looking for an authentic orchestral performance or a unique percussive layer for your next pop track, here is everything you need to know about castanet VSTs. Why Use a Castanet VST? castanets vst