On kicks or changes of possession, all blocking below the waist is prohibited.
Clipping has always been, afaik, below the knees from behind. In the open field, on a normal run, think of the face of a clock, the blocker needs to block the defender between 10-2 on the clock face, with 12 being dead center on his body.
There are provisions for times when the defender turns himself from the blocker after he has committed to the block that causes contact outside this area so that the blocker isn't penalized for what would have been a legal block.