Insert allows you to move the pivot point of an object so what Kurt is most likely doing is making sure his pivot point is on the center axis.This is done by first selecting the move tool then hiting insert (you will see the center of the tool change to a little circle) then you middle mouse drag over the center of the grid while holding down X ( MMB + X lets you snap to grid), once the pivot point is where you need it you hit insert again to complete the move.You can now do your mirror action across the X axis knowing the pivot is in the center.