Concentration is required on this path of yoga, for all poses, but the degree of concentration both physically and mentally needed for the bow is most high. The emotional release following this can be intense and I highly recommend this on melancholic days if emotions start to jam. To concentrate means to put all of one's attention and effort on that one thing. The definition for "concentrate" as a noun also beautifully applies: to reduce, to extract diluting agents; to become smaller so only the essence is left.
Power concentrates and narrows down to its raw essence, it doesn't blow itself up, strut around and puff about.
The physical concentration takes place in the solar plexus or manipura chakra located in the lumbar region in the spine where the kidneys are in the back and around the belly button in the front. To keep the power concentrated, rather than passive or scattered, focus on the solar plexus region in the front and corresponding part in the back. It's from this region that the legs sprout up. My arms are there to guide, not to pull and yank my feet. The power radiates from the middle outward, not from hands, arms or shoulders towards the spine. The feet and legs should be higher than the head, and there should be the sense that the hips are higher than the head. The forehead should only cautiously rise up once the power out of the solar plexus area has stabilized.
To prepare, lie flat on your tummy with one arm in front, the other back along the thigh. Stretch the front arm forwards on the ground and lift it up, while also lifting the opposite leg and back arm on the left. Hold the lift and gather focus on the solar plex at which point the spine will rise. The spine will lift not just up, but also outwards. The spine lengthens out towards your head and front arm, and also lengthens in the back toward the back leg and arm. After a few breaths, lower the spine and forehead to the ground, and change your arms. Repeat on the other side, and continue to alternate the arms at least two more times before bending your knees and grabbing the outside of your feet to prepare for the bow.