It attaches to the back of the Nerf top rail and makes the cocking mechanism much easier and more ergonomic to pull. It's especially more ergonomic when an accessory--red dot sight, flash light, camera--is attached to the top rail. The cocking operation becomes much quicker and smoother. Not to mention it looks awesome as well.
You will also need a #6-32x1-1/4" screw and a #6-32 nut to fasten this mount to the Nerf tactical accessory rail. The screws and nuts are not included, but can be easily sourced from your local hardware store (e.g. Home Depot, Lowes, etc.). You may also order them through the following Amazon links: