don't understand slur control-points

[Stefan Thomas]

Dear community,
I have a problem with a slur, which doesn't look nice, as You can see in the below quoted snippet.
Off course I know, that I can change the ceontrol-points. I tried to understand their meaning, but without success.
Could someone, please, give me a hint?
Thanks, Stefan

\version "2.13.0"

re = { \change Staff = rechts }
li = { \change Staff = links }
reru = { \re \stemDown }
lira = { \li \stemUp }
music = { 
  \once \override TupletBracket #'padding = #-7.5
  \set tupletSpannerDuration = #(ly:make-moment 1 4 )
  \times 2/3 { \li g 8 (\re e' \li bes \re  d'' g' f'' cis''  ais'' fis'' ~ \once \override TupletBracket #'padding = #-2 fis'' \li f'! b ) |  } }
\new PianoStaff { 
  << \new Staff ="rechts" \music 
    \new Staff = "links" s1 >>
    
  }