From this post, I've learned that the leading edge flap works by increasing the camber of the airfoil. But how exactly does it work? Since it is a flap, I can guess it works by changing the chord line, just like the trailing edge flap does. But how exactly does the chord line change? Does the leading edge move down to below the original leading edge when the leading edge flap is extended?
Leading edge flaps and droops do increase the camber because when they move, the leading edge actually droops, and with it the leading edge extremity of the chord line, while the mean camber line remains in place for most of the airfoil length:
The airfoil is dramatically bent down to the point the bottom side is now concave and the top side has a larger convexity.
Effect on the lift coefficient:
For the same angle of attack α, CL is decreased, but it is now possible to reach larger α and with them larger CL. It also means lower speeds are required to generate the same amount of lift.
LE flaps are usually found at the outboard portion of the wing.
More: Aircraft Design, a conceptual approach, Daniel P. Raymer (around page 278 - view it)