Foreshortened is  the description of our view of a body or object placed perpendicularly or at oblique angles to your level of vision.  The object appears to be compressed.  An arm is called foreshortened when the model's arm is raised and turned toward the artist.  Any body with volume will create an impression of foreshortening.  Any object will always have one part or another foreshortened, no matter how they are viewed.

Foreshortening has been a subject of much study and research by artists.  Museums and private collections contain hundreds of drawings, usually preliminary studies of foreshortened hands feet heads and arms. 

Foreshortening presents more difficulty to the artist than almost any other part of drawing.  If you were to point at yourself with your right hand, with your forefinger at exactly eye level and then try to draw the hand, you would have to rely on highlighting so that it is obvious that it's a finger. 

A sculptor can mold the hand realistically, but an artist must reproduce the hand on a flat surface,  such as canvas or paper without actually being able to reproduce the hand's depth.  In order to accomplish this, the artist would have to draw in all the subtleties of shade and light in order to create the illusion of relief or depth. 

When you look at your own finger pointed at you, you have no problems recognizing it or its position because you can see it coming straight out of your hand toward you with the fingernail in the foreground.  In spite of complete foreshortening, your eye can run over its entire length.  But the artist works with just two dimensions and he can't reproduce bodies as he sees them in relief.  He works on a flat surface on which the third dimension, depth, does not exist.

For the student who is in front of a model for the first time, this creates a real difficulty.  He has been accustomed all his life to see in three dimensions.  His brain and his eyes are so used to positioning the different parts of a given object at a different distance, that even when he looks with only one eyes, he will continue to see in relief. 

When the student finds himself confronted with a model, he sees it in three dimensions which he tries to transfer to the paper and finds himself completely at a loss, unable to reduce what he sees in three dimensions to only two. 

Copying a drawing or photo is a straightforward matter because the difficulty of three dimensions doesn't arise.  In a drawing the features are flat.  There are no problems of foreshortening or depth.  Everything can be measured in terms of height and width.  It is simply a question of transferring these measurements with the correct interplay of light and shade to achieve the same results.

If you want to break the foreshortening barrier and the two dimensional barrier, you must learn to see the subject without the third dimension.