The G-dliness of Keilim. It is easier to relate to the reality (Metzius) of Keilim than to Ohr, as Keli is a defined Metzius while Ohr is P’shitus. Thus the meeting of Ohr and Keli, though both are G-dly is the meeting of two incompatible things and inevitably affects a Tzimtzum.
Page 476-8. NEW SV p. 628-30. MP3