In Yoreh Deah 339:1, the mechaber brings a variety of things that one is not permitted to do to a goses (a person on the brink of death), since each of these things causes the goses to die sooner. Since a goses is considered a living person in every respect (הגוסס הרי הוא כחי לכל דבריו) even one who closes their eyes as they are dying is a murderer (וכל המעמץ עם יציאת הנפש ה"ז שופך דמים).
Some of the examples that are given, concerning what may not be done, are of a curious nature, but the strangest (to my mind) is one brought by the Rema:
וכן אסור לגרום למת שימות מהרה כגון מי שהוא גוסס זמן ארוך ולא יוכל להפרד... אסור לשום מפתחות ב"ה תחת ראשו כדי שיפרד
Likewise, it is forbidden to cause the deceased to die quickly - for example, if he has been a goses for a long time and is unable to depart... It is forbidden to place the synagogue keys [or the keys of the bet hamidrash] underneath his head [the head of the goses] in order that he may depart.
Seeing as none of the nos'ei keilim appear to comment on this, and since I have never heard of such a thing anywhere else, I wonder if anybody here can shed light on it? For what reason was it believed in the 16th century that placing the keys of the synagogue (or bet hamidrash) underneath a sick person's head might cause them to expire? Does this have a basis in any older examples of rabbinic literature?