The idea here is that the man has an animated walk-cycle and wherever his foot intersects the plane it will stick.
By adding the inverse displacement of whatever foot is on the ground to the man's position, the foot remains fixed, and the man moves forward
I made this demo as a (very) quick demonstration of an alternative to matching a walk-cycle with a movement speed to avoid "sliding" along the ground. This way the walk cycle controls the speed of the player to create a realistic look.
Unfortunately you have to download the Unity plugin (~30MB last time I downloaded it) to see this, and it's *really* buggy, but again, it was only a proof of concept.