[…] He learned all there was To learn about not launching out too soon And so not carrying the tree away Clear to the ground […]
The boy starts to get better about swinging the trees over time.
He learns to get all the way to the top of the tree and not bend it too soon, before he's reached the top.
If he did jump out too soon, the tree would be damaged.
If you're a science person, think of this as a Physics lesson combined with a Biology lesson: the tree is a flexible lever; the roots are the fulcrum; the boy is the load.
[…] He always kept his poise To the top branches, climbing carefully With the same pains you use to fill a cup Up to the brim, and even above the brim.
Now we're getting some details of how the boy becomes better at swinging the trees.
He keeps "his poise," meaning he stays balanced and calm, sort of hovering up on a tree branch.
The speaker compares it to filling a cup to the brim. If you are pouring liquid into a cup, you are so careful not to overflow the cup, so you add a small amount of the liquid at a time. Then you add just a teeny bit more and the liquid forms a dome just above the rim of the cup.
Think of this one as a Chemistry lesson. Are you beginning to notice that nature and science play important roles in this Frost poem?
Then he flung outward, feet first, with a swish, Kicking his was down through the air to the ground.
The boy has filled the metaphorical cup above the brim and has now reached the top of the tree.
Next he kicks his feet out (presumably holding onto a branch) and uses the tree like a bungee chord.
The tree bends just enough so that the boy is lowered to the ground without harm.