Probic vent

The probic vent was an essential part of all cloned Sontarans.

Information
At least partially artificial, the probic vent took the form of a small metallic tube attached to the back of the Sontaran's necks. This vent was normally used by the Sontarans as a way of directly feeding off energy to "recharge" rather than eating (as the latter was slower). Raw plasma was fed directly into the vent to nourish them. (DWM: The Betrothal of Sontar)

The vent was also a major weakness, since a blow to the probic vent could knock a Sontaran unconscious or even kill them. (DW: The Time Warrior) A lesser blow could cause them pain and/or stun them. (DW: The Sontaran Stratagem) An arrow (DW: The Time Warrior) or a thrown knife (DW: The Invasion of Time) would almost certainly prove deadly.

The Sontarans did not consider it a weakness however, since it meant that they must always face their enemy in battle and could not be cowardly and retreat. (DW: The Time Warrior, The Sontaran Experiment) Strangely, some Sontarans had an extra part on their helmets that covered the probic vent. (NA: Shakedown)

However, some people occasionally found ways to disable Sontarans from in front of them. For example, the Doctor momentarily disabled General Staal using a squash racquet and a ball by rebounding the ball from Staal's teleport pod and onto Staal's probic vent. (DW: The Sontaran Stratagem) Donna Noble used a hammer to hit a Sontaran on the probic vent when it had its back turned. (DW: The Poison Sky)

The Doctor used the same hammer in 2010 to stun Jask who was about to shoot Mickey Smith and Martha Jones who were working as freelance alien hunters at the time. (DW: The End of Time)

The energies directed through the vent seem to be at least partly electrical in nature; When Chrissie Jackson jammed the metal tip of one of her high heels into Kaagh's vent, she received an electric shock. (SJA: The Last Sontaran)

List of weapons used to strike a probic vent

 * A metal pole. (DW: The Time Warrior)
 * A bow and arrow. (DW: The Time Warrior)
 * Knives. (DW: The Invasion of Time)
 * A squash ball. (DW: The Sontaran Stratagem)
 * A hammer. (DW: The Poison Sky, The End of Time)
 * A high-heeled shoe. (SJA: The Last Sontaran)
 * Sword. (DW: A Good Man Goes to War)