Hunting optics manufacturer Leupold didn't just come out with a new rangefinder, they created a whole new category: the bow mounted laser rangefinder. The new Leupold Vendetta rangefinder mounts to the riser on your bow and can be used when you are at full draw. No you don't look through it, that was my first thought too, it actually has a red LED screen on the back of the unit that displays the range so you can easily see it during hunting situations.
Sighting In
The Vendetta rangefinder unit has a visible laser that is used only for sighting in purposes and is not used during hunting. To sight in the Vendetta you use your top sight pin and adjust the visible laser until it is ranging parallel with your top pin. Since, your top pin cannot be in perfect alignment with the Vendetta, if it was you couldn't see your pins, it will always be off by the amount of offset from your top pin to the Vendetta rangefinder. This distance should only be a few inches, and as long as you follow the sight in procedures and have the rangefinder aiming parallel, this will be close enough for aiming the rangefinder in the field.
Ranging
The new Leupold rangefinder has a short cable lead coming from the base of the unit and that cable has a trigger mechanism on the end. This trigger button is mounted on the handle of your bow so that you can use it to activate the rangefinder while at full draw. This rangefinder also has a scan mode so once you have drawn your bow you can follow your target as it gets closer and the range will be constantly updated, no need to press the button twenty different times just hold it down and the yardage will update on the red LED screen.
Notes
The Leupold Vendetta is not legal in all states for hunting purposes, so be sure and check your state's laws or visit the Leupold website to see if it is legal for hunting in your area. As of this writing many online stores had these units listed with the majority of them asking 0; however, none of the stores had them in stock. So like with many new hunting optics these might be difficult to find for awhile.