strange things you should probably avoid doing with the pipeline

PipelineToggleExample.ps1

function FooBarPipe{
   param([parameter(ValueFromPipeline=$true)][int]$val)
   begin{
      $reset = (get-item Function:\FooBarPipe).ScriptBlock
      $Foo = {
         param([parameter(ValueFromPipeline=$true)][int]$val)
         begin{set-item Function:\FooBarPipe -Value $bar -Force}process{write-output ('foo {0}' -f $val)}
      }
      $Bar = {
         param([parameter(ValueFromPipeline=$true)][int]$val)
         begin{set-item Function:\FooBarPipe -Value $foo -Force}process{write-output ('bar {0}' -f $val)}
      }
      set-item Function:\FooBarPipe -Value $foo -Force
   }process{
      write-output (FooBarPipe $val)
   }end{
      set-item Function:\FooBarPipe -Value $reset -Force
   }
}

1,2,3,4,5 | FooBarPipe