I'm not sure I understand what you mean...

The mirror is always up during Live View, tethered or not. The only way to display a live image on the LCD (camera or computer) is with the mirror up.

There are three different types of focusing in Live View - Quick AF, Live AF, and Face AF. Quick AF flips the mirror down to use the dedicated phase AF system (used with the viewfinder), Live AF is contrast AF from the imaging sensor, Face is Live AF with face detection. If focusing is causing the mirror to flip (down then up), you're using Quick AF (not what you want).