Q) Are there any other situations in which p’sik reishe is permitted on Shabbos?
A) We have previously established that although p’sik reishe is generally prohibited on Shabbos (Shulchan Aruch OC 337:1) it is mutor to perform a p’sik reishe d’lo niche lei of a melocho d’rabonon in a situation of tza’ar (Mishna Berura Siman 316 s.k. 5 quoting the Chaye Odom).
We find another leniency regarding p’sik reishe on Shabbos:
The Remo tells us (OC Siman 316:3) that one must take care when closing the door of a small cupboard that there are no insects trapped inside – since if there are insects inside, by closing the cupboard door one will be inevitably trapping the insects (p’sik reishe of tzod).
The Mishna Berura infers from the Remo that the only prohibition is to close the door of a small cupboard; but there is no prohibition to close the door of a large cupboard – even if insects will be inevitably trapped inside the cupboard. The reason that this is permissible, explains the Mishna Berura, is because in a large cupboard it would not be possible to trap a small insect with one lunge, and therefore this situation of tzod is only prohibited midrabonon. Additionally, insects and the like are creatures that are not typically trapped (i.e. to use or to keep) and therefore these species are not subject to tzod min hatorah – only tzod midrabonon. The Mishna Berura explains in Sha’ar Hatziyun (s.k. 18) that since there are two independent reasons that this action is d’rabonon and not d’orayso – it is mutor in a p’sik reishe situation.
We see from here, that it is sometimes mutor to perform a p’sik reishe (d’lo niche lei) action on Shabbos – if there are two independent factors that render this action d’rabonon and not d’orayso.
8 אבל ברור מתוך דברי השעה”צ בס’ של”ז שרק דרבנן של המלאכה עצמה (כמו הכא אין במינו ניצוד וא”א לתפשו בשחיה א’) מתירין (במצב של פ”ר דלנ”ל) את המלאכה, ולא דרבנן שהוא בגלל חסרון מלאכת מחשבת (כמו מקלקל, מלאכה שאצ”ל, שינוי וכד’).
9 כמו שחלקנו למעלה בין דרבנן של המלאכה עצמה לבין דרבנן שהוא בגלל חסרון מלאכת מחשבת