Headshot Only is a room option limited to 4 maps. This option forces players to score a headshot to kill opponents - shooting/stabbing any other part of the body results in no damage whatsoever. It is very useful for training badge, since players can ensure that all they get is Headshot, with no normal kills mixed in.
One of the most infamous trick / glitch abusing in this mode is to face a wall and put the crosshair down, or look at the ground at a certain angle. That way, the player's head will bend into the wall, giving opponents less room to register a hit. It is useful to
Troll and irritate players
Distract enemies and help your teammates
The above trick can also be useful to force opponents to not shoot you or risk getting a 18_1 client error (Shoot through wall hacks)
In some versions, there is a glitch that lets you play HS only on every map. (Random map option)
If shot at the neck, you deal 29 damage to the player as Crossfire considers the neck portion as a 'head'.