Bicycle technology improves in teeny, tiny increments. So it's a once-in-ten-years thing when a device comes along that might reliably shave 20 seconds in just 3km. But that's exactly what the Cranklock purports to accomplish--and the invention makes obvious, intuitive sense.
A bit of background. If you're on a bike and you're cornering, you can't put any weight on your inside foot---if you do, the pedal obviously goes down, thus putting you in danger of scraping the ground. Wrecks often result--and that's why pro bikers put their weight on the outside pedal during tight cornering. (Watch the rider's pedals in this video if you're confused.)
But think about that for a second. Putting your weight on the outside means you're basically forcing the bike vertical--and, of course, straight. You're not carving as tight a corner as you otherwise might. By contrast, motorcycle riders lean into a corner. Pro racers nearly touch their inside knees to the ground doing so.
Okay, now that's out the way: Cranklock does just what the name suggests. At the push of a button, it locks the cranks in place, so that you can distribute your weight between inside and outside pedal any way you want. Thus, you can carve tighter corners, and slingshot around them--and that can be save 20 seconds in just 3km of aggressive descending. Which is truly a massive amount--in the Tour de France, for example, the entire race is sometimes won by just a few seconds.
The gadget was initially designed as a safety measure to eliminate speed wobbles. But it's proven useful in all kinds of settings, such as BMX, where rotating pedals can cause gruesome wipeouts.
[Update: There seems to be some controversy over the physics involved. Care to weigh in?]