I'd be happy to explain! :) It works best on similar body types as the curve will be most similar to begin with (which they are in this genre of art frequently). If you take an image with a relatively smaller body and align one hip, then scale up, you'll get a point where both sets hips are the same, at least for a short distance.
The idea is founded on the fact that two points determine a line. If you choose two points in an "copied" picture (hip 1 and hip 2), you will always be able to line them up with the same two points in the "original" to some degree of success. (If you have a line segment it only has 3 properties: position (which is accounted for by moving the pictures in the x-y direction), angle (which is obtained through rotation), and length (which is obtained through scale).)
Now that handle getting the basic features in place, but there are curves. That can be accounted for by having the angle and position be slightly off. Because the hip is a large curve, it means that if the curves don't fit together quite perfectly when the hips are in exact alignment, they can be rotated a bit and still line up well (in comparison a sharp angle where there is one definite point that is or is not in alignment). You can actually try this by drawing a parenthesis and an angle bracket on a sheet of paper and the same on a sheet of tracing paper - the parentheses will have a lot more give while still being "lined up." Again, this works best in a similar pose and/or with a similar body type, but you'll be able to find more overlap than might be expected in hips.
no subject
The idea is founded on the fact that two points determine a line. If you choose two points in an "copied" picture (hip 1 and hip 2), you will always be able to line them up with the same two points in the "original" to some degree of success. (If you have a line segment it only has 3 properties: position (which is accounted for by moving the pictures in the x-y direction), angle (which is obtained through rotation), and length (which is obtained through scale).)
Now that handle getting the basic features in place, but there are curves. That can be accounted for by having the angle and position be slightly off. Because the hip is a large curve, it means that if the curves don't fit together quite perfectly when the hips are in exact alignment, they can be rotated a bit and still line up well (in comparison a sharp angle where there is one definite point that is or is not in alignment). You can actually try this by drawing a parenthesis and an angle bracket on a sheet of paper and the same on a sheet of tracing paper - the parentheses will have a lot more give while still being "lined up." Again, this works best in a similar pose and/or with a similar body type, but you'll be able to find more overlap than might be expected in hips.
I hope that all made sense. :)