A probic vent is a small hole at the back of a Sontaran's neck, used by the clone race as a plug socket to recharge their power levels.
During his encounter with the Third Doctor, Space Commander Lynx of the tenth Sontaran battle fleet explains that the positioning of the probic vent - which is extremely vulnerable to attack - is a strength, because it means Sontarans must always face the enemy.
This is disproved on just about every encounter The Doctor has with Sontarans, as they are seen time and time again being incapacitated with a blow to the probic vent.
The Sontarans became generally less and less interesting as they went on in Doctor Who when actors other than Kevin Lindsay played the warrior race, meaning we even got cockney Sontarans in The Invasion of Time and culminating in the Colonel Blimp cyphers we got in the rubbish The Sontaran Stratagem.
The last time we saw the Sontarans it was in the last few minutes of The End of Time, fittingly showing a Sontaran being disabled with, you guessed it, a blow to the probic vent.
This excellent spot of toy photography below comes from an amazing Flickr set by Decepticreep and shows a Sontaran being murdered by the Sixth Doctor, who is plunging a sonic lance into the Sontaran's vulnerable flesh.
It's a fitting image in that it displays perfectly the fundamental design flaw in the probic vent, and also the explicit violent tendencies of Colin Baker's incarnation - both of which are on ready display in The Two Doctors.