If the eel was split open from the belly side, the flesh on the edges would break apart and fall off the skewers, therefore it was necessary to split open from the back side so that the edges would be thick enough.
腹から裂いた場合蒸す過程で外側の身が割れて串から外れてしまうため、外側が厚くなる背開きが必然でもあった。