M. Brindos
Hey, no problem!
Undercutting, or relief cutting, is when you use a x-acto, scriber, or very fine razor saw to score along the edges of the different items on the figure, such as collars, belts, cuffs, etc.
It gives the different objects some much needed depth and separation. When done well it gives a more 3D look to the figure and adds realism.
The bigger the figure, especially 1/35, the more realism you can manage to squeek out of the figure. I don't really bother with 1/72 and in 1/48 there is little you can usually do. But that depends on the quality of the figure and the detail that was sculpted into it.
These new Airfix figures are pretty well done and detailing them does make them shine a bit more so they are worth the extra efforts.
I hope that explains it well enough. :)