I have often come across, in writing on cross-country skiing, the importance of the compression phase of poling. One is advised, on pole plant, to compress, contracting the abdominals to initiate the action of poling. Let’s compare the action of poling with the action of the arm against the trunk in free-style swimming. The latissimi dorsi (lats) are the primary muscles that extend the shoulder against the trunk in swimming and in cross-country skiing. In swimming, it is essential to lengthen the trunk while pulling oneself through the water with the hands/arms. First, because it is important to keep the trunk long and hydrodynamic, secondly, because the powerful lats must be opposed by the muscles which extend the spine to utilize their full potential. To compress and contract the abdominals would take the lats out of action, and would transfer the work of shoulder extension to anterior muscles and arm rotators, so the work of shoulder extension would be limited by the relatively small mass of muscles like pectorals and teres major. In other words, the work of the abdominal muscles would be limited by the mass and strength of muscles that connect them to the shoulder for extension. In order for the lats to be effectively engaged, it is important to lengthen the trunk on pole plant, so that the spinal extensors support and oppose the action of the powerful latissimi dorsi.
Here is a photo of a skier beautifully extending her body from head to toe, and poling without compression: