Confused by Piano Dynamics

[Ryan M. Rifkin]

I'm trying to parse the piano-dynamics example (for the 1.4 series,
posted last month).  I'm confused by this section:

dynamics = \notes {
  \outputproperty #(make-type-checker 'dynamic-interface)
    #'extra-offset = #'(0 . 2.5)
  s2\fff\> s4 
  \outputproperty #(make-type-checker 'dynamic-interface)
    #'extra-offset = #'(0 . 2.5)
  \!s\pp
}

Specifically, what are the skips (s's) doing in this context?  Taking
out the first s2 causes a syntax error.  Making it into s16 doesn't
affect the place of the \fff, but seems to make the descrendo shorter.
Changing the second s to an s2 seems to make everything a little
shorter.  I'm sure this is something obvious, but I just don't quite
get it.

Thanks,

rif