avoid slur help

[james]

I'm having some difficulty understanding the 'avoid-slur property.  
This doesn't work:
\version "2.11.65"

\relative c'' {
   \override TextScript #'avoid-slur = #'inside
   c2*1/2( s4^\markup {\natural} d4.) c8
}

In fact, it increases the space between the slur and the notes, but  
doesn't move the markup down.