Resurrection device

The resurrection device was a piece of alien technology that could, depending on the severity, temporarily or permanently bring people back from the dead. It was used by Vastra and Jenny Flint to heal Strax' wounds after the Battle of Demons Run. It was later used by Strax in an attempt to save Clara Oswald after she was pulled off a cloud and again to bring Jenny Flint back to life when she was killed by a Whisper Man.

Capabilities and limitations
According to Vastra, the resurrection device could bring anyone back to life "for a while". It was never shown exactly how serious Strax's injuries were in the battle of Demons Run, but the resurrection device succeeded in reviving him. Vastra believed he had made a full recovery, although the Eleventh Doctor thought that Strax might have lost some of his intelligence suggesting that the technology might have a negative effect on a person's mind. Jenny thought that Strax had never truly died, saying she thought he had just fainted, which most likely helped the technology keep him from passing away since it didn't have to actually pull him back out of death. (WC: The Battle of Demons Run: Two Days Later, TV: The Snowmen)

Once somebody had actually died, the resurrection device could revive them but there was no guarantee that they would recover. Jenny Flint recovered when she was killed by a Whisper Man. Her death was presumably caused by stopping her heart since that was their signature method of killing. All the resurrection device had to do was get her heart pumping again, a relatively simple task. (TV: The Name of the Doctor) However eventually, the device would run out of power and, if the dying person hadn't recovered by the time it did, the person would die. The device's success in keeping people alive depended on the exact severity of the injuries. Clara Oswald had broken many of her bones and injured her internal organs after falling off a cloud and the technology didn't have enough power to heal such severe injuries, only being able to bring Clara back to life for a short time. Despite Strax's best efforts to save her, Clara eventually died. (TV: The Snowmen)