One explanation of the Gemara regarding the machlokes Tana Kama and Rabban Gamliel concerning one who finds tefilin on Shabbos. Rashi’s addition that is difficult to understand. Rashi is explained with a yesod from Tosfos, and another Gemara. This drasha is part of a longer drasha that the Beis Yitzchak zt”l learned with his son R’ Aharon zt”l when he became bar mitzva. His son passed away in his lifetime and the drasha was published in his sefer Beis Yitzchak (O.C. 19). At the beginning he writes a request that lomdei Torah and yerei Hashem should learn this drasha with their sons. This is a drasha that contains a powerful message – how careful one must be not to transgress an issur.