Probic vent
The Probic vent was a part of all cloned Sontarans. At least partially artificial, it 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). 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 find it a problem, however, since it meant that they must always face their enemy in battle and could not be cowardly and run away. However, some people occasionally found ways to disable Sontarans from in from of them: for example, the Tenth Doctor momentairily disabled General Staal using a squash racket and a ball by rebounding the ball from Staal's Teleport pod and onto Staal's Probic Vent. (DW: The Sontaran Stratagem)
