The probic vent was a tube on the back of a Sontaran's armour.
At least partially artificial, the probic vent took the form of a small metallic tube entering the back of the Sontaran's neck. This vent was normally used by the Sontarans to feed directly on energy to "recharge" rather than eating. Raw plasma was fed into the vent to nourish them. (COMIC: The Betrothal of Sontar)
The vent was also a major weakness. A blow to the probic vent could knock Sontarans unconscious or even kill them. (TV: The Time Warrior) A lesser blow could cause them pain and/or stun them. (TV: The Sontaran Stratagem) An arrow (TV: The Time Warrior) or a thrown knife (TV: The Invasion of Time) would almost certainly prove deadly.
Sontarans did not consider this a weakness however, since it meant that they must always face their enemy in battle, nor could they be cowardly or retreat. (TV: The Time Warrior, The Sontaran Stratagem)
As Commander Strax revealed in his field report, the prospect of a glorious battle for the Eleventh Doctor and "the Impossible Boy" had made him so excited he clogged his probic vent. He suggested that this was an example of fun. (WC: Strax Field Report: A Glorious Day)
Weapons used to strike probic vents included a metal pole, a bow and arrow, (TV: The Time Warrior) knives, (TV: The Invasion of Time) a squash ball, (TV: The Sontaran Stratagem) a hammer, (TV: The Poison Sky, The End of Time) a high-heeled shoe, (TV: The Last Sontaran) a sword, (TV: A Good Man Goes to War) a loose wooden plank, marbles fired from a slingshot (GAME: The Gunpowder Plot) and various metal implements used for sport, such as darts and a javelin. (PROSE: The Sontaran Games)