This is an exercise that we learn in the basic repertoire. At first glance it looks like an easy exercise until you break it down and THINK about what it does to YOUR body. The reason I ask for you to think about it personally is beacause we are all different shapes and sizes. Therefore the stretch someone may feel in the hamstrings others may feel in the glutes, stomach or ribcage.
In his book Return To Life Joseph Pilates gives these directions:
Spread legs as wide apart as possible
Draw toes pointed upward and backward
Rest palms on the mat
Then with outstretched arms , palms flat on the mat
Chin to chest
Begin reaching forward with three successive sliding motion-stretching movements as far as possible.
Taken from Pilates'Return To Life Through Contrology
In the Spine Stretch Forward exercise you lift the ribs up off the pelvis, reaching the legs away to create space between the top of the pelvis and hip. Scooping to help find the correct position.
Here is an image with a great teacher Kathryn Ross Nash of The Red Thread Ranch showing what the Spine Stretch Forward should look like.
Reprinted by kind permission of Kathryn Ross Nash.