When you're climbing a hill on a full-suspension mountain bike, you get considerably more pedalling efficiency if you lock out the rear suspension. A lot of riders don't bother, though, especially since they might forget and leave it locked afterwards. That's why the Killswitch was created. It works with the bike's existing dropper seatpost (assuming it's got one) to automatically lock out the shock, but only when climbing.
The Killswitch is essentially a spring-loaded cable that extends from the base of the seatpost to the lockout lever on the shock.
When the post is in its middle or lowest settings (for riding on flat terrain or going down hills, respectively), the spring is compressed and the lever gets pushed to the Open position. Once the post is raised to its highest setting (for climbing hills), however, the spring is released and the lever gets pulled to the Closed position.
So far, the prototypes work with certain models of RockShox shocks. If a just-launched Kickstarter campaign is successful, though, the hope is that there will soon be Killswitches compatible with almost all makes and models. Shocks with levers that have three settings may also be accommodated.
If you're interested in getting one, you can do so for a pledge of US$95. Delivery is estimated for December, if all goes according to plans.
And should you not want another add-on cluttering up your bike, BMC's Speedfox mountain bike now features similar functionality built right in.