Vehicle power-trains use a variable transmission (multiple gear-ratios) to minimize motor size and maximize efficiency while meeting a wide-range of operating points. Robots could similarly benefit from variable transmission to save weight and improve energy efficiency; leading to potentially groundbreaking improvements for mobile and wearable robotic systems. However, variable transmissions in a robotic context leads to new challenges regarding the gear-shifting methodology: 1) order-of-magnitude variations of reduction ratios are desired, and 2) contact situations during manipulation/locomotion tasks lead to impulsive behavior at the moment when gear-shifting is required. This paper present an actuator with a gear-shifting methodology that can seamlessly change between two very different reduction ratios during dynamic contact situations. Experimental results demonstrate the ability to execute a gear-shift from a 1:23 reduction to a 1:474 reduction in less than 30ms during contact with a rigid object.