Guardian dual function provides both steady on & flashing modes just by reversing the batteries (2-CR-2016 lithium coin cell batteries provided). Patented reverse polarity switch. Just clip it on and go. Lightweight, yet tough and durable, it will not restrict movement and may be attached anywhere. Waterproof up to 300?, works from -40?F to ??, twist lens to turn on or off, visible over 1 mile, high impact and scratch resistant.