It isn't snake oil, or magic, it is based on a very basic mechanical principle of lever length. Inside the box is a lever on a pivot, the cables entering either side can be installed in different position to vary the leverage by changing the relationship of the clutch cable to the short link cable. In fact they can be used either way round.
So you can reduce clutch release movement and reduce the pressure at the handlebar lever (the design intent), or increase the required pressure at the handlebar and increase the clutch release movement (my intent).
I have one here that I want to use to increase the movement of the clutch pushrod to help with a clutch drag problem I have on a Norman White Belt Drive and TTi gearbox combination. I think the problem only exists because of the specific combination, but Brooking850 had similar issues with another belt drive and the TTi box and his chosen bar lever.
I bought mine through 24MX in France, cheaper than the US link our French friend Marin Atlas posted. They are popular on modern off road bikes to reduce lever pressure ( I think mainly for Enduro where fatigue is a bigger issue than in my 20 minute road races! )
www.24mx.fr