This paper presents a swift and agile untethered insect-scale soft propulsor inspired by the *Stenus comma* beetle. The propulsor, with a body length of 3.6 mm, achieves a recorded specific speed of ~201 BL/s and acceleration of ~8,372 BL/s², surpassing previous designs by several orders of magnitude. Key features include a bioinspired magnetic-triggered tail for on-demand fuel delivery, a precise surfactant release mechanism, and a momentum transfer-induced braking system. The propulsor demonstrates on-demand motion with high acceleration, velocity, and deceleration, enabling agile braking and complex trajectory planning. The work offers new insights into the design of artificial insect-scale soft robots, potentially revolutionizing robot design, optimization, and control paradigms.This paper presents a swift and agile untethered insect-scale soft propulsor inspired by the *Stenus comma* beetle. The propulsor, with a body length of 3.6 mm, achieves a recorded specific speed of ~201 BL/s and acceleration of ~8,372 BL/s², surpassing previous designs by several orders of magnitude. Key features include a bioinspired magnetic-triggered tail for on-demand fuel delivery, a precise surfactant release mechanism, and a momentum transfer-induced braking system. The propulsor demonstrates on-demand motion with high acceleration, velocity, and deceleration, enabling agile braking and complex trajectory planning. The work offers new insights into the design of artificial insect-scale soft robots, potentially revolutionizing robot design, optimization, and control paradigms.