Femto indicates smallness. Extreme smallness. Like, the size of a proton small. While the Lezyne Femto Drive light isn't quite that tiny, it is very unobtrusive. And, it's made to last. CNC-machined from a chunk of solid aluminum with a composite-matrix rear cap that doubles as a mount, the Femto Drive will take abuse and is very user-friendly.
Pushing the Femto's lens will turn it on and off, and cycle through it's four flash modes also. It's rear cap/mount fits on your seatpost, and is secured with a custom-molded silicone strap to fit a wide variety of shapes and diameters.
The Femto comes with two CR2032 batteries.