At least I think that's what this technique is called! You simply stamp your image twice & leave one uncoloured. On the second image, colour in the portion of it you want to highlight and then cut that out and superimpose it on the first. In this case I used a Nesties circle die to cut out the highlighted portion.
This is how the finished card looked, it was for my cricket-made nephew's 23rd birthday.
Cricketer stamp by Craft UK, sentiment stamp Penny Black Salutations, backing paper by Deja Views.