Like most tiny LEDs, this is a visibility aid rather than a replacement for your main rear light.
On your bike, it will fit to a seatstay, seat rail, cable outer or, as pictured, a rack strut. But it’s best for fitting to the kind of flagpole you might have sticking up from a trailer – which must have its own light(s), by law, for UK night riding – or recumbent. Not only does it fit securely, at 22g with battery it won’t bow the pole over either. Fitting is by a painfully tight rubber ring and a contoured plastic cleat. The single red LED (white version available) is powered by a 3 volt CR2 battery (included) for a claimed 150 hours. Light is steady only, not flashing, and you need to be careful switching it on: turn the body one way, the light comes on; turn it the other, it comes apart so you can get the battery out.