In pilates there is a focus of two placements of the lumbar spine (very low back) and pelvic region. These two placements are the neutral or imprinted positions.
It should be clear at all times in pilates whether you are in neutral or imprint. There are several occasions when you might be in a passing through position, in the event of spinal articulation.
The pelvic region holds the largest percentage of the body’s weight load; therefore, it is important to grasp the idea of stability and mobility in this region.
Neutral Pelvic Position
Neutral position is the most stable and shock-absorbing position that we can put our pelvis and lumbar spine in; therefore, it is pilates class but also in our daily life.
the ideal position for us to be in, not only in When you see a person lying (stomach facing up) on the floor in neutral position often you will see a slight curve in the low back, this is normal. Naturally, anatomically, we have a slight curve in the lumbar region.
However, to find your own neutral do not judge by the amount of space between your low back and the floor. (Look below for instructions on how to find neutral pelvis.)
Imprint Pelvic Position
This position is generally used for certain body types and is also used in the beginning to ensure stability of the pelvis and lumbar spine region if neutral position cannot be maintained. This position is also great for stabilizing weak abdominal obliques, with neutral being the ideal placement goal.
Often when you see a person lying down in imprinted position you do not see a space between their lumbar region and the floor. It is important to note that the low back is not jammed into the floor; rather it is lengthened almost parallel to the floor.
(taken from http://thinkpilates.com/pilates-basic-principle-pelvic-area/)