Never heard of this practice - 'shield their eyes and look away'. First, if you are holding your musket correctly, your right cheek would be 'welded' into the left of the stock. Virtually impossible to 'look away'. What you may be referring to is the practice of 'closing' your 'aiming' eye (usually right one) just as you pull the trigger so that any powder residue from the flint lock mechanism would not be blown back into your open eye. Burning powder residue in your eye would make it very difficult to continue to load and fire.