Who's The Smarter One Now?

After several days of digging, it crossed Siful's mind why he and Naj had to do all the hard work while A sleeps blissfully under the shade of a tree, and posed the question to Naj, who replied "I don't know. Why don't you ask A?" And that, was precisely what Siful did.
After climbing the long ladder out of the trench, Siful confronted A. "I want to know why Naj and I have to do all the hard work while you sleep all day long, and have a better pay than us?"
Knowing that Siful is not too bright a person, A decided it would be better to show rather than explain. As he stood up beside the big tree he said "See my left palm here? I want you to punch it as hard as you can. If you manage to do that, I'll go down into the trench and do your work while you can sleep all day long". A then put his left palm just in front of the tree.
Not wanting to miss an opportunity, Siful threw the hardest punch of his life. A, being a man of better experience, pulled his palm just before the punch landed, and instead of the palm, Siful punched the tree, and let out a scream of pain.
After bandaging Siful's right hand, A said "Now you know why I am here and you are down there? I am smarter than both of you put together. Now, go back to your work".
Siful went back down the trench. But before he could continue his work, Naj asked him "So, do you have the answer already?" And a smile lit up on Siful's face.
"Yes,"Siful said. "Now, I will show you why I am smarter than you. But first, you must punch my left palm as hard as you can". Being in the middle of the trench and far from its walls, Siful put his left palm in front of his face.
-/end.
ps. Read related story at Rocky's Bru and here.