The reset button was a component of the Doctor's TARDIS next to the phone that could home the TARDIS in on the Doctor's watch if the TARDIS "burped" and hopped forward in time.

Sally Sparrow learnt from her future self how to operate this control and send the TARDIS from 2005 to the Ninth Doctor in 1985. After doing this, in a video recording, she told herself from two hours in the past how to operate the reset button, explaining that she told herself about this. (PROSE: What I Did on My Christmas Holidays by Sally Sparrow)

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