פירוש על ברכות 36:2

To harden his body in order the better to master the intricacies of the Torah (= slay a lion).

On the Sifra see p. 71 n. 6. It is regarded as the most difficult of the Halakic commentaries on the Pentateuch and is therefore called "a lion." He mastered this work on a winter's day, i.e. in a short space of time.