Foreshortening literally means to show figures and object as they appear to the eye when seen from an oblique angle. Actually foreshortening is part of a large field called the law of perspective which is the diminishing of an entire object when it is seen obliquely. To be able to use foreshortening in a hentai drawing you must first know something of the rudiments of the laws of perspective.
First, almost everything that is seen by the naked human eye is foreshortened. Only if an object is seen at a direct right angle will it appear to be one dimensional all other give the appearance of distance. When drawing any part of the human figure using this perspective the artist must change the dimension of the figure relative to how close or far it is from the observing eye. Simply put closer objects are drawn larger than those parts of the anatomy that are further from the observer.
When a hentai figure is drawn, their relative position to each other and the reader is indicated by ‘stretching’ the closer image and diminishing the picture the further it is away from the reader’s point of view. If a woman is bent over with her breasts hanging down while she is having sex from behind then the swaying breast should be larger in one frame as the move forward, then smaller in the next as they move backward. If a tentacle is moving out of a demon’s back or chest then it should appear to elongate away from the figure and in the next panel, move towards the girl from her perspective.
It is this ‘trick’ of illustrating such items from frame to frame that creates the illusion of depth to a drawing.