The purpose of this exercise is to establish the correct sequence of movements of the hands as they move away from the body on the recovery. The objective is to have the right hand closer to the body than the left hand and the knuckles of the right hand in the palm of the left hand at the cross over.
The sculler sits at backstops and extends the left arm fully, and then only follows with the right, aiming to slot the right hand into the palm of the left. The sculler then draws the right hand in and when it is against the ribs draws the left hand in. The sculler then repeats this. Once the coach sees that the sculler is starting to get the order quite naturally they can begin to row both hands moving together always ensuring that the left hand leads away and the right hand is closer to the body on the recovery and that the right hand draws in first on the drive.
It is probably best to do this exercise at a spot where steering is not crucial as the boat will zig-zag quite a bit, but should generally go in a straight line.