An approach in the gemara to explain the argument between the Tana Kama and Rabban Gamliel regarding one who finds tefillin on Shabbos. Rashi’s addition that is difficult to understand. Introducing a yesod of Tosfos in the Din of Ba’al Tosif.  A further gemara. In light of the above, Rashi can be understood. This drasha is a small section of a larger drasha that the Beis Yitzchak zt”l learnt with his son, R’ Aharon zt”l when he turned Bar Mitzva. It is printed in his great sefer, “Beis Yitzchak”, Orach Chaim 19. He begins the drasha by requestion that lomdey Torah and yirei Hashem learn this drasha with their sons. At the end of this drasha there is a powerful message that relates to chinnuch and to being careful to avoid issurim