[…] then fell to right awayNicking and slicing neatly, heaving sodsOver his shoulder, going down and down (21-23)
This snippet shows how strong the speaker's grandfather is (mentally and physically) – he hardly takes a breather from his work.
Between my finger and my thumbThe squat pen rests. I'll dig with it. (29-31)
The speaker has decided that he is going to be just as skilled with his tool (the pen) as his father and grandfather were with their spades. But what do you think he'll be digging up with his pen?