Synestic locks were a form of clamp used on Earth spacecraft to hold the vessels securely to their take-off platforms. Before a ship could launch, its locks had to be manually removed by someone on the platform. If the space vessel was set for automatic take-off, removing the final lock would trigger the engines and kill whoever removed it.
When the transport ship on Nerva was overrun by the Wirrn swarm, the crew programmed the ship to launch into space and then disembarked. The Fourth Doctor and Rogin proceeded to remove the synestic locks and when the Doctor tried to sacrifice himself by removing the last one, Rogin knocked him unconscious and dragged him to safety before taking the final lock and perishing in the process. (TV: The Ark in Space)
As the Doctor removed his first lock, he absent-mindedly stuck the hand-sized device into one of his pockets. This would save his life; when the Sontaran warrior Styre shot him with his hand weapon, the lock absorbed most of the energy and melted. The Doctor noted that this was why he never threw things away, since "you never know when these bits and pieces will come in handy". (TV: The Sontaran Experiment)