This is a classic patch where a complex sound source – such as one oscillator frequency modulating another – is sent through a Low Pass Gate with either just a trigger to “strike” the vactrol inside or otherwise an instant attack/fast decay envelope to create a nice percussive sound. The fact that the low pass gate reduces the higher harmonics as its volume dies away helps tame the harmonics coming from the complex source, and give it a decay similar to a struck percussive instrument.
