When the spade sinks into gravelly ground: (4)
Right away, Heaney dives in to talk about the difficult labor of digging.
Till his straining rump among the flowerbeds (6)
The word "straining" in this line shows us how hard his father was working in the garden. Perhaps things no longer come so easy to his dad.
The coarse boot nestled on the lug, the shaftAgainst the inside knee was levered firmly. (10-11)
Here, our speaker's dad is working in the potato fields, and from the word "firmly" we can see that he has a skilled grasp on his tools.