I have "heard" a lot about this here at Armorama, but found no good explenation about what this technique is all about.

Someone that would like to explain and tell me what it can be used for.

Preshading is the same as underpainting, from what I know, it's to enable a "shadow /toned" effect. Before the main color is applied/sprayed. We usually spray a darker tone like black covering all the recessed/ joints/ detailed areas, then spray the main color to overlap. Because the "preshaded" color is darker than the main color, naturally there'll be 2 tones, instead of one featureless color.

Modelers here use it for just about everything.
Good example would be especially fighters, check out the links for better explanation

http://www.clubhyper.com/reference/preshadingrf_1.htm

http://us.games-workshop.com/games/40k/imperialguard/painting/treadheads/p2/2.htm

http://www.finescale.com/fsm/community/forum/topic.asp?page=-1&TOPIC_ID=27026&REPLY_ID=388564

Thomas, Nick is essentially correct, but it's important to realise that everyone has there own favourite method of preshading. Mine is as follows. If I am painting a vehicle such as a WWII German AFV, I will first spray an all over coat of either matt black or another dark colour such as Tamiyas' Nato black. Over this I will apply a coat of e.g. XF60, dark yellow. Initially this coat will be very light, almost a 'dusting' effect. Bring the airbrush closer in to the model I will begin to spray a heavier coat in the centre of panels and around panels etc. This will leave a graduated effect, with the centre of panels being the proper dark yellow, fading to a much darker at the edges of the panel. If, when finished the effect is too marked then bring the airbrush to about 5 or 6 inches from the model and give another light dusting to all the surfaces until you achieve the effect you want.
Hope this helps.

Vinnie

Thomas, one 'trick' with pre-shading, is to undercoat in a contrasting shade. For example, German Sand. I am mucking about with pre-shading of dark red oxide, which gives a superb basis for painting...Jim

Why does everything need an airbrush...

you do not need an air brush to acceive the same effect. It can be done with a brush but you have to be careful not to go too heavy with the paint when overcoating.

Most of the shading done on my models is with a brush and only the main coats of paint and clear gloss or matt are AB'ed on.

Here's another form of pre-shading. First an undercoat of matt black is sprayed (I use the cans of Games Workshop black)

Next stage is a 'misting' of Russian Geen (very dark)

Next is a mixture of RG and Panzer Yellow

Finally, White + RG...

This ISN'T finished, i'm now beginning to buildup the color and effect I want..Jim