Professor Kalu claimed to have created the chronon seed, which she isolated in order to be able to measure interstitial time. According to the Fifth Doctor, a chronon seed should be a theoretical impossibility.
After operating their chronon bomb, and being projected into the Time Vortex together, Chris Jennings was in possession of Kalu's chronon seed. He gave it to Tegan Jovanka to keep her and the Doctor from leaving in the TARDIS. It allowed the Doctor and his companions to travel back, once, to microseconds before the explosion without ill effect. When they did so, time was in complete stasis around them. (AUDIO: Interstitial)