שור עביד לנתוקי - It is usual for an ox to become untied
Summary:
רש"י maintains that שור דרכו לנתוקי on its own accord; however תוספות maintains that (since it is קשור כראוי) it is דרכו לנתוקי on account of the חש"ו, as opposed to a גחלת which tends to become extinguished.
ולרבי יוחנן דאמר אפילו מסר לו שלהבת פטור כולי -
And according to ר"י who maintains that even if he gave over a torch to him he is פטור, etc.
Summary:
The reasoning behind the dispute between ר"י ור"ל is that ר"י maintains that a חש"ו add somewhat to the שמירה; while ר"ל maintains that the חש"ו (generally) diminish the שמירה.