Ask HN: Software slave interface?

In Being Popular, PG says:

  One solution here might be to design systems so that interfaces are 
  horizontal instead of vertical-- so that modules are always vertically 
  stacked strata of abstraction. Then the interface will tend to be owned by
  one of them. The lower of two levels will either be a language in which
  the upper is written, in which case the lower level will own the interface,
  or it will be a slave, in which case the interface can be dictated by the
  upper level.

I think I understand what a language interface looks like, but I do not understand what a slave interface looks like. Can anyone here provide any good examples?