Using shape memory alloys (SMAs) as actuators offers several advantages, including high power density, large stroke capabilities, and friction elimination. These attributes make SMAs suitable for actuating flexure-based mechanisms in a wide array of extreme environments. However, their extended cooling time limits their use for applications requiring a high cadence or fast commutation. This paper presents the design of a novel type of mechanical actuator conceived to address this dynamical performance issue. The proposed mechanism is a bistable switch based on a preloaded buckled beam attached to a flexure pivot. The mechanism is actuated by a set of individual modules, each powered by an SMA spring. Each module is selectively engaged and disengaged from the switch through a selection gear put in motion by a coupling blade. Since only one module is active at any given time, the others can cool down in the meantime: the commutation time is hence reduced by a factor of compared to a classical antagonistic-pair SMA design. This solution operates passively: it doesn't require active cooling or any additional energy intake. This innovative and generic design is intended for use with SMA actuated bistable mechanisms, leveraging their advantages to achieve very high accuracy, precision and repeatability while keeping a highly dynamical performance.