Mirror modifier clip toggle

Yesterday I was at the local bookstore and was reading a book on game character creation with blender. In this book, the author was modeling out a human character and with mirror modifier attached, swapped in between clipping on and off several times in the creation of his model. I am kind of curious about this approach, especially clip in the MM is something I turn on and forget (in my earlier models, I ended up with a crease in the middle of my models because of this when applying a subdivide mod ).

What am I missing with this tool and what can you accomplish from Clip on/off? Thanks and Happy New Year, everyone!

Turning off clipping just allows you to move vertices off the mirror axis, that’s all

ah ok- otherwise they are locked in place if they are on the axis then?