Q. ( Re- question 1327). Thank you for your comprehensive answer on using an electronic key-card on Shabbat. I have two more questions. You mentioned that there is at least a rabbinic prohibition for using the card. What exactly is the offense based on?

A. Besides the known opinion of the Chazon Ish, that activating an electric circuit is boneh and constitutes a Biblical prohibition, there are different views and opinions in the Poskim.
Beis Yitzchok (Y.D. 2: 31: index) and others maintain the proscription of closing and activating an electric circuit is molid (Making changes to an object or substance. Creating a new entity on Shabbos). He writes: Creating a current flow (molid zerem) is rabbinically forbidden because in doing so one has created something new – a functioning appliance. (See Minchas Shlomo pp. 71-74; Tzitz Eliezer 1:20:10, Tz’lach Hachadash, Kuntres Acharon 1).
Poskim also assert that activating any electrical equipment involves “makkeh bepatish,” literally, striking with a hammer; an act of completing an object and bringing it into its final useful form. These Poskim cite as precedent those who prohibited winding a watch for this reason. (Chazon Ish, O. H. 50: 9; Mishp’tei Uziel 1: 13; Tzitz Eliezer 6: 6; Edus Leyisarel (Rabbi Y. E. Henkin) p. 121. (Rabbi Henkin states that perhaps only a Rabbinic prohibition is involved. The prohibition of metaken mana, is a sub-prohibition of makkeh bepatish).

Rabbi A. Bartfeld as revised by Horav Shlomo Miller Shlit”a