When a ball is punted, the return specialist wants to gain some yards to give the team good field position, but if there’s no way he can safely field the ball due to the punt coverage, he signals a fair catch by waving his arms over his head. At that point, nobody can touch him. Failure to head the fair catch signal is a 10 yard fair catch interference penalty. There used to be a penalty called a “halo violation” – the halo was a two yard bubble around the returner, and if a defender entered that zone, even without touching him, it was a penalty. Fortunately it was done away with because it was pretty arbitrarily enforced. A defender who had no intention of touching the punt returner could be penalized and give the receiving team ten additional yards just for being in the wrong place on the field. Honesty, I never understood how anyone could expect the defenders to run full speed down the field and then be able to stop on a dime to avoid the halo anyway. The caveat is that the fair catch signal has to be clearly made. If a returner doesn’t use both arms or have them over his head, there is no penalty called if he gets hit.