法華義疏 7.176

7.176

「宜加用心無令漏失」者，用心謂順法隨機，無令漏失謂勿違理差機。密表大法將為已任，故有此誡也。

Regarding "You should apply your mind diligently, and not let anything be lost": applying the mind means conforming to the Dharma and according with capacities; not letting anything be lost means not contravening principle or missing capacities. It esoterically indicates that the great Dharma is about to become his own responsibility; hence this admonition is given.