The techniques Heaney uses in this poem are onamatopoeia "Squelch" for example.
Also in the second last verse Heaney uses a listing device.
Also he uses lieration "curt cuts" whichgive added ethisis.
Heaney also uses roots to describe his family roots.
Heaney is effectively "digging" his memories at the instant he begins to write, he looks out his window and starts reminiscing about his past, he then becomes enthralled in guilt because he did not follow his grandfather, and fathers path, which leaves him feeling guilty and below them, he feels his forefathers were great men and he is not, however he still speaks of his pride in his own choice in a "pen is mightier than the sword" kind of way
This poem compares his life to that of his father. He looks at the skill of his father and sees that he can never do what his father is so skilled at “digging”. He describes in fine detail the art and mechanics of digging potatoes and the ease at which his father can do it.
He compares the skill of his father digging potatoes to that of his grandfather digging in the bog, a completely different skill. Again he described in fine detail the art of digging turf and the sights and sounds that are part of it.
The wonderful line that shifts time lines from the present , where he looks at his father perhaps clearing up a gravel path “comes up twenty years away” to him digging potatoes.
In the second last verse a line from each generation is compared and tells him that his that his only skill is to “dig with his pen”
He knows that he can never be as good with a spade and feels that he is more comfortable with a pen. (snug as a gun)
Repeating the lines in the last verse confirms this comfort and confirms his only way to match