Takes one quality of the colour from the second picture and gives it to the first, leaving everything else alone.
Colour can be described as three separate things: which colour it is, how strong that colour is, and how bright it is. This splits both pictures into those three and swaps just one across.
That is a genuinely different operation from blending. A blend mixes two colours together and gives you something in between. This substitutes one quality outright and leaves the other two exactly as they were.
Take brightness from the second picture and you get the first picture's colours wearing the second's light and shade. Take the colour itself and you repaint the first picture without touching its shading at all, which is how you recolour something and keep it looking solid.
The slider runs from one picture, through the recombination, to the other.