Question about jungling

I'm new so I'm asking here. My question is why do junglers attack a monster and then move back then attack and move back again? I am watching nightblue right now and level 2 he did it to red buff, attack move back attack move back etc. I know whatever he is doing is a good thing as he kills the monster and take minimum damage but then again I don't know what hes doing and when I try to copy him I take more damage than I normally do. Also with rengar he can do a full clear but I can onlt do a half clear, the only difference is that he does the technique mentioned above and I don't. Is there perhaps a video explaining what he's doing and why its good, if so please link it to me. I have watched a few "jungle guide" videos but non of them explain the attack move technique on monsters.
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