Subsurf modifier mirror->snap left/right together? [solve

In a tutorial video (za06b_andy.avi) Andy edit a subsurf with the mirror modifier to model a pretty animal thing. At one time he let the left and right side of the top head subsurfes snap together. I didn’t get that, how does it work?

The red lines are drawn by hand to show where I want to snap the vertices together.

Aah, solved!

Clicking “Do Clipping” in the mirror modifier will do that!