His shoulders globed like a full sail strung Between the shafts and the furrow. The horse strained at his clicking tongue. (2-4)
You can see the physical effort the father is giving by his domed shoulders; even the horse is working his tail off. He's got to be one strong dude to have shoulders that huge, too.
An expert. He would set the wing And fit the bright steel-pointed sock. (5-6)
Not only is he strong, but he's skilled, an expert with the plowing equipment. You can't have one without the other, it seems. The consonance of those T sounds gives the line a neat and exact feeling, which is perfect to show us how expert el padre was.
The sod rolled over without breaking. (7)
This line attests to the father's skill and strength; you have to have a pretty delicate touch to plow through the ground without breaking it. It's like being able to fell a giant oak tree then whittle it into a perfect sculpture. There's no limit to this guy's awesomeness.
At the headrig, with a single pluck Of reins, the sweating team turned round (8-9)
The father is not only skilled at working with the plowing equipment, but he's also skilled at dealing with his team of horses. Is there anything this guy can't do in a field? Heaney uses this example of the team's obedience to echo the son's admiration.
[…] His eye Narrowed and angled at the ground, Mapping the furrow exactly. (10-12)
The job takes concentration and mental strength as well. The father's focus is part of the recipe that makes him a great plowman.