The analogy 'his shoulders like a big sail draped between the shafts and the furrow' emphasizes Heaney's impression of him as a strong and huge boy. Heaney tells how he gets the horses to move as a "team" and turn around as he utilizes his talents to map "the furrow accurately" in the third verse. A sail strung along a ship's mast provides air circulation below deck while allowing sunlight through. Sails are the large, flexible membranes used for floating on water or in the air. On a boat, they are attached to a framework called a masts which extends out from the hull. The ends of the masts hold the sails, which can be made of cloth or plastic. When the wind blows, it pushes against the sail, making the boat go forward. People use sails as an analogy for a powerful person because sails represent strength. They also represent freedom because you can go anywhere with the wind at your back. Heaney uses this image to describe Michael as a young man because he was a big, strong boy. Sailing is a popular activity in Ireland so this image fits well with the surroundings. Michael was born near Beltaine when people celebrate the beginning of summer by dancing around a maypole. So, using sailing as an analogy, he was able to show what kind of influence he had over others even at a young age.