Yeah sorry, that mechanic is not clearly explained, my bad. Here's what I told another commenter who had the same problem:
The idea is to grip with the first hand near the pile (either left or right hand is fine, as long as the other hand makes contact with the pile during the rotation), and when the second hand is over the pile, press and hold the button to grip with the second hand. Release the button for the second hand to throw the rock.
You can also use the skip level button or level select to check out the other levels if you want :)