Two versions of Alistair Gordon Lethbridge-Stewart short out the time differential between themselves. (DW: Mawdryn Undead)
The time differential was an aspect of multiple points in a single timeline which had come into contact with one another.
When a human met him or herself from another point on his or her own timeline, the Blinovitch Limitation Effect meant that physical contact between the two versions of the person would short out the time differential between them, and could result in dangerous effects such as a large energy discharge or damage to time itself. (DW: Mawdryn Undead, Father's Day) In some cases, however, the ill effects of two versions of a human shorting out the time differential between themselves could be avoided or suppressed. (DW: A Christmas Carol)
- How or why Kazran Sardick could touch his younger self without the ill effects of shorting out the time differential remains unknown.
The Fifth Doctor appeared physically older when meeting his future self. (DW: Time Crash)
While multiple incarnations of the same Time Lord encountering one another did not experience dangerous effects of shorting out the time differential between themselves (DW: The Three Doctors), the earlier incarnation or incarnations would appear of a greater physical age than they should have at that point in their timeline. This otherwise harmless effect would disappear once the incarnations parted ways and the time differential was no longer shorted out. (DW: Time Crash)
When time itself became stuck and began dying as a result of River Song altering a fixed point in time by refusing to kill the Eleventh Doctor, multiple points in Earth's timeline came into contact with each other as a result. The only way to repair the damage to time was for River and the Doctor to physically touch; as their personal timelines were the polar opposite points of the distortion, their contact shorted out Earth's own time differential. This enabled them to return to the fixed point, restore the required event, and repair the damage to time. (DW: The Wedding of River Song)
Behind the scenes
Originating in Mawdryn Undead, the concept of "shorting out the time differential" was used in Time Crash by Steven Moffat to finally put an on-screen explanation to the unavoidable fact that the previous Doctors in multi-Doctor television stories are visibly older than they appeared when the actors had last played the Doctor on-screen.