What exactly does retake do?

All the other editing options seem relatively self explanatory. Retake does not have hardly any options, and it's not obvious what it's for. I thought it would be the generalist option for things not covered by the other ones (like adding audio to an existing video without changing it, stuff like adding a character walking in from the side that was not there before, or things like changing characteristics like hair color without full replacement while leaving the rest alone), but when I try using it the output seems almost completely divorced from the input. A bit more explanation on exactly w