When does the act of donning tefillin end?

Hi,

When we put on tefillin, we put on the arm tefillin, then we put on the head tefillin, and then we wrap the straps of the arm tefillin around our fingers. However, the mitzvah act is complete once the arm and head tefillin are on, before the wrapping around the fingers.

Therefore: The act of putting on tefillin is complete once one puts on the head tefillin, and after that one may respond "Amen" to a berachah one hears. However, one should not respond "Amen" until after saying the "Baruch Shem Kvod" line, since that is meant to be joined with the berachah on the head tefillin.

(Rivivot Ephraim 1:30:1)

Have a great day,
Mordechai