Which is the trick behind this picture?
Which tecnique did they use?
Than you

https://www.google.co.uk/search?q=an+le … B500%3B375

There are some different techniques in this series of images...and especially in the one you feature.

It seems like the overall saturation of the skin has been reduced somewhat... to give you that porcelain/non saturated look to the skin.

It seems that the tonality of the skin has been changed by adding in some brown to the overall skin color.  This can be accomplished with an adjustment layer with the red channel pushed up a bit, the blue layer pulled down a bit, and the RGB layer pulled down a bit more to give you that brown tonality...and then... it was selectively painted on by turning the mask basically black...and then painting in the brown tones with a very soft, low opacity white brush.

The skin looks nicely retouched generally... with blemishes and blotches and shadow transitions harmonized or removed.  This is done generally with the healing and cloning family of tools in Photoshop.

Some of the dark black areas have been made a blue/black tonality.  You can use several ways...but one way is to use adjustment layers with curves.  Take the blue channel is curves and lift up the left bottom point of the blue curve to make the blacks become contaminated with a blue color tint or tone.  You could also deepen the blue bu also adding in a red curve slid right at the left bottom part of the curve to get a steely kind of blue.  Use the RGB part of the curve by pulling it down in the center to darken up the blues ALOT.  On this adjustment layer...you can also use a blending mode of darken and especially multiply to really get your blues to become sexy.  Now all of your dark parts of the image are really contaminated deeply with a dark blue cast...so invert the adjustment layer mask to become black...the blueness will disappear...then creatively and artistically paint in the blue tones where you think they will be sexy with a very low opacity soft white brush.  Its the choices you make and the creativity and artistry you bring...that will make your image look cool and modern and sexy.

It also looks like the lightness and warmth have been bumped up in the mid tones of her brown hair.  Add in some red and lightness and paint it in with several obvious warming and lightening techniques (hue /sat, curves, levels, D+B on color channels for example).

DONT EVER DISCOUNT...the value of: a great looking model, a professional makeup artist on staff, obvious great hair stylist in this image, a wardrobe person getting a perfect designer quality dress for this shot, a stage construction artist who gets you the perfect moody wall paper for this set... and a totally awesome $300 rental sexy chair to go with the wallpaper and dress.  You also better be a lighting designer too... to get those great moody shadows and an evenly lit face. 

It could be that this image is a team event possibly... with a cast of 4-6 experienced professionals... possibly from an agency... with a good budget!  And OH YES...Your retouching client wants you to recreate this same image quality and look...but he has:  a one person team, an inexperienced TF weekend photographer, no $800 lighting rigs , a plain dress, a free model from Mayhem who didnt show up the first time, and no makeup person,  and no hair stylist.  I dont think its possible for a retoucher to give you a quality image like this...without all the pre-production details and people being good quality first!

Cheers. 

There are always 10 ways to skin a cat in Photoshop...But you and your client wont get the same kind of quality results together if the original image was not produced well from the beginning!

Good lighting to begin with is the trick