This multi-functional tool generates and gives out ultra sonic impulses which cause vibrations in anything to which the tool is applied. In addition to its intended purpose to drive screws, the kinetic energy of the these vibrations can activate any simple mechanical apparatus. It particularly useful for opening both mechanical and electronic locks. By boosting its power and concentrating the sonic energy in a beam, it can even be used to set off mines. Reversing the polarity turns it into a powerful electromagnet. Although useful the sonic screwdriver is not infallible.
The Doctor usually kept this tool about his person, until it was unfortunately destroyed by the Terileptils in the seventeenth century. At some point prior to the Master's trial and execution by the Daleks, the Doctor obtained (probably built himself) a replacement.